According to Trinitarians who believe Philippians 2:6 says Jesus is God, why did Paul add the word 'form' ('morphe')?

In the Wartburg Project blog (for Lutheran EHV Bible translation), there is a 2017 blog article answering your question exactly In Philippians 2:6 should "morphe" be translated "form" or "nature"? which makes the argument that the Greek word morphe is often translated by the Latin word forma. But the Latin forma and English form are false cognates. The English term nature (or even essence) is better for conveying the meaning of the passage. See quotes from the article below.

The article cited the book The Two Natures of Christ by Martin Chemnitz a second-generation German Lutheran theologian (1522-1586):

Damascenus tells us that according to the usage of the ancient church, the terms essence or substance (Greek ousia), nature (phusis) and form (morphe) are synonyms and designate the same thing (Two Natures, p 29).

The well-known passage in Phil. 2:6–9 is also pertinent here. It deals with “the form of God,” “the form of a servant,” “the likeness of men,” and “the high exaltation of Christ.” The “form of God” by the unanimous testimony of the ancients is the divine nature or essence itself, according to which Christ by nature is equal with God, but not by robbery, such as Satan and Adam attempted. Furthermore, the term “form” (morphe) is used to designate a nature or essence endowed with peculiar attributes and conditions, divine or human, which is covered and ornamented with them, so to speak, as Augustine says … ad Petrum, “You must understand the ‘form of God’ as the natural fullness of God.” (Two Natures, p 326).

Quotes from the article about the false cognates leading to a wrong understanding:

At the heart of the problem is that the Greek word morphe is often translated by the Latin word forma, and forma in turn is often translated or glossed by the English word form. The problem is that Latin forma and English form are false cognates. In dogmatics forma almost never means form. It usually means essence. The statement that “the forma of Scripture is the divinely intended meaning” cannot properly be translated “the form of Scripture is the divinely intended meaning.” It must be translated “the essence of Scripture is the divinely intended meaning.” Form and essence here are contrasted. In English, the “form” of Scripture refers to the outward form, the letters, the words, the sounds of the words. Even when the form (the Latin word is materia not forma) of Scripture (the sounds, the shape of the letters, even the language) changes, the essence (the divinely intended meaning) remains the same. That is why accurate translations convey the Word of God.

Since Latin forma and English form are false cognates, English readers are apt to read the word “form” as something less than the full divine nature. The term nature (or even essence) is better for conveying the meaning of the passage. Since the common English usage is to speak of the two natures of Christ, the term nature commends itself here, and it has no doctrinal downside as form does.

Overview
Philippians 2:1-11 may be divided into three parts, each with a different temporal focus; each using different terms for what Trinitarians believe is the divine triadic; each with a word which connects the flow of Paul's message; each with its own result; each with one who takes action:

Paul begins by calling upon the believer's fellowship of the Spirit and then says not to act in self-interest or in vainglory. The effect is to have the mind of Christ Jesus.¹A key term in the flow of the passage is κενοδοξία, a hapax legomenon which describes two characteristics, κενόω and δόξα. These two are significant in the next two divisions. κενόω, in the second division is self-emptying. δόξα, in the third is glory.

The morphē of God
The most common use of morphē is describing form or appearance. The term is immediately repeated: Jesus took upon Himself morphē doulos, the appearance of a slave.²Paul continues to say Jesus was found in human form, which reinforces the idea Paul is speaking of appearance. In the Old Testament, the appearance of God was often described as the glory of the LORD. This meaning of morphē is consistent with what is said elsewhere:

He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. (Hebrews 1:3)

The meaning of glory agrees exactly with what Jesus prayed before His death:

⁴ I have glorified You on the earth. I have finished the work which You have given Me to do. ⁵ And now, O Father, glorify Me together with Yourself, with the glory which I had with You before the world was. (John 17)

In His prayer, Jesus speaks of glory He had with the Father before the world was which was before Jesus was on the earth and it speaks of glorifying the Father, which is how the passage in Philippians ends. Therefore, in terms of appearance Paul states:

Jesus had appearance of God [glory with the Father before the world was] of which He emptied Himself in order to take on the appearance of a slave. After His death on the cross, God exalted Him [returned the glory of which He emptied Himself]...to the glory of God the Father

God exalted Jesus by giving Him the name that is above every name, so that at the name of Jesus every knee should bow, in heaven and on earth and under the earth, and every tongue confess that Jesus Christ is Lord. When this happens, it will be to the Father's glory. That is, the glory which Jesus and the Father had before the world was.

Who Takes Action
An important aspect of the entire passage is who is it that acts. Paul begins by telling the Philippians they need to act; they are to think like Christ Jesus. Paul ends by stating how God acted: He exalted Jesus. In the middle section, it is Jesus who acts.

Jesus became in the likeness of man and He became obedient to death on the cross. In both cases, Paul uses the verb γίνομαι, as does John in the Prologue, the Word became flesh (1:14) and as in the Gospel, Paul uses the middle voice. The middle voice in general means the subject performs or experiences the action in such a way that emphasizes the subject's participation.³

Both of Paul's uses of morphē are done without making mention of God or the Father taking any action. By purposely omitting all other references, the effect is to say Jesus acted by Himself. In other words, the passage has been composed to say Jesus had the appearance of God and at the same time, by doing things only God can do, Jesus acted like God.

Conclusion
Paul used morphē to mold the passage as a Trinitarian would describe the Son of God becoming human for the purpose of dying on the cross. By using morphē Paul makes the point Jesus had something which was quite real before He took on a different morphē to be found as human.

It is a way to describe the past using a term which demands pre-existence before His death on the cross and it has been done in a way which states not only did Jesus have the appearance of God, He did things which only God is able to do.

<sub>1. The Greek is τοῦτο φρονεῖτε ἐν ὑμῖν ὃ καὶ ἐν Χριστῷ Ἰησοῦ. The term, φρονεῖτε which is translated as mind is a verb which means to think upon. Literally, Paul is saying to think like Christ thought.
2. morphē doulos, the appearance of a slave follows Paul's use of doulos in Romans (cf. 6:16-19). All, except Christ, have sinned and are slaves to sin. When Jesus was seen in human form, He had the appearance of a sinner, morphē doulos.
3. Daniel B. Wallace, The Basics of New Testament Syntax, An Intermediate Greek Grammar, Zondervan, 2000, p. 182.</sub>

ος εν μορφη θεου υπαρχων ουχ αρπαγμον ηγησατο το ειναι ισα θεω

Philippians 2:6 [TR - undisputed]

... who, being in the form of God, thought it not robbery to be equal with God ... [KJV]

... who, being in the form of God, thought `it' not robbery to be equal to God ... ... ... [Young's Literal Translation]

... who in [the] form of God subsisting, not rapine esteemed it to be equal with God ... ... [Englishman's Greek New Testament - literal]

First as to Paul's use of morphe. Paul uses the same word when he says to the Galatian church, Galatians 4:19 : 'I travail in birth again for you that Christ should be formed (morphe) in you.

Clearly, this is not a matter of outward appearance but of indwelling spirit.

God is no respector of persons, says Peter, Acts 10:34. So, also, saith Paul in Romans 2:11.

Here, in Philippians 2:6, the Son of God demonstrates, as reported by his own appointed apostle, that even he, himself, does not base his outlook on his own person. He does not regard his own divine person and then make estimations, or assumptions, based upon his own person.

Rather, being himself in a specific nature (morphe), or 'form', as some suggest the translation should be, he regards his own nature/form and, therefore (and only therefore) esteems that he is equal in deity. He has a certain form, or nature, and on this basis, he regards that he has equality. That is to say equality 'with' (KJV) or 'to' (YLT) God.

He does not seize equality based upon his own person.

It is perfectly understandable why the verse is so constructed.

It could not be otherwise.

For God is whom he is.

This verse very clearly declares both the divine nature of the Son of God, Jesus Christ, and the divine person of the Son of God, Jesus Christ.

Baptised into the Church of Scotland at the age of five and baptised, as an adult, into the Baptist Assembly of Scotland, at the age of sixteen, I have been a lifelong Trinitarian and have studied the records of the Council of Nicaea and have studied the Trinitarian views of such as :-

Athanasius (297-373), Augustine (354-430), Columba (521-597), John Wycliffe (1328 -1384), Martin Luther (1483-1546), John Calvin (1509-1564), John Knox (1514-1572), John Owen (1616-1683), George Whitfield (1714-1770), William Huntington (1745-1813), Robert Mussay McCheyne (1813-1843), William Gadsby (1773-1844), J C Philpot (1802-1869), John Kershaw (1792-1870), J N Darby (1800-1882), John Burgon (1813-1888), Robert Young (1822-1888), William Kelly (1821-1906), J K Popham (1847-1937), Herman Hoskier (1864-1938) and John Metcalfe (1931-)

All express the same view on this particular passage. There is unanimous agreement.

There can be no doubt that in classical Greek it describes the actual specific character, which (like the structure of a material substance) makes each being what it is; and this same idea is always conveyed in the New Testament by the compound words in which the root “form” is found (Romans 8:29; Romans 12:2; 2Corinthians 3:18; Galatians 4:19). (3) On the other hand, the word “fashion,” as in 1Corinthians 7:31 (“the fashion of this world passeth away”), denotes the mere outward appearance (which we frequently designate as “form”), as will be seen also in its compounds (2Corinthians 11:13-14; 1Peter 1:14). The two words are seen in juxtaposition in Romans 12:2; Philippians 3:21 (where see Notes). Hence, in this passage the “being in the form of God,” describes our Lord’s essential, and therefore eternal, being in the true nature of God; while the “taking on Him the form of a servant” similarly refers to His voluntary assumption of the true nature of man.

Ellicott's Commentary for English Readers

Notice the two natures of Christ; his Divine nature, and human nature. Who being in the form of God, partaking the Divine nature, as the eternal and only-begotten Son of God, Joh 1:1, had not thought it a robbery to be equal with God, and to receive Divine worship from men.

Matthew Henry's Concise Commentary

The importance of the passage on the question of the divinity of the Saviour will be perceived at once, and no small part of the point of the appeal by the apostle depends, as will be seen, in the fact that Paul regarded the Redeemer as equal with God. If he was truly divine, then his consenting to become a man was the most remarkable of all possible acts of humiliation. The word rendered "form" - μορφή morphē - occurs only in three places in the New Testament, and in each place is rendered "form." Mark 16:12; Philippians 2:6-7.

Barnes' Notes on the Bible

The word rendered "being" (ὑπάρχων) means, as R.V. in margin, being originally. It looks back to the time before the Incarnation, when the Word, the Λόγος ἄσαρκος, was with God (comp. John 8:58; John 17:5, 24). What does the word μορφή form, mean here? It occurs twice in this passage - Ver. 6, "form of God;" and Ver. 7, "form of a servant;" it is contrasted with σχῆμα fashion, in Ver. 8. In the Aristotelian philosophy (vide ' De Anima,' 2:1, 2) μορφή. is used almost in the sense of εϊδος, or τὸ τί η΅ν εϊναι as that which makes a thing to be what it is, the sum of its essential attributes: it is the form, as the expression of those essential attributes, the permanent, constant form; not the fleeting, outward σχῆμα, or fashion.

The Pulpit Commentary

ὅς] epexegetical; subject of what follows; consequently Christ Jesus, but in the pre-human state, in which He, the Son of God, and therefore according to the Johannine expression as the λόγος ἄσαρκος, was with God.[92] The human state is first introduced by the words ἑαυτὸν ἐκένωσε in Php 2:7. . . . . it simply narrates the former divinely glorious position which He afterwards gave up: when He found Himself in the form of God, by which is characterized Christ’s pre-human form of existence.

Meyer's NT Commentary