According to Jehovah’s Witnesses why did John write the words found in John 1:1?

The reason why is because they like other Christians believe that the context of John's whole gospel is Jesus and his role in salvation. One JW said it this way to me: John wanted to establish Jesus as his agent for creation as well as his instrument for salvation. He wanted to make it clear that Jesus existed in the presence of God as an angel or (lesser god) before he came to be a man on earth.

Now, you're right. It certainly is peculiar for a first century Jew to use words akin to hierarchal duo-theism while his contemporaries were strictly monotheistic. Why didn't he just say out right that he was an angel? That certainly would have cleared up the issue. John like other jews believed in one God and that no other god was formed after him or before him (Isa. 43:10) making this interpretation of John 1:1 problematic.

Having debated many JW's myself over this verse, they find comfort in their logic for the lack of the article making theos indefinite rather than definite, but when applying the same logic to other indefinite thoes's in the same chapter and even in the same book, they do not apply the same logic. In other words their are many places where thoes lacks the definite article but they do not render those passages as 'a god'. Why the inconsistency? In my opinion, it's theological bias.

Jehovah's Witnesses believe John included John 1:1c in his gospel account because his gospel account was inspired by God and necessary for learning the truth about Jesus. The inclusion of John 1:1c fulfills prophecy and helps set the context for the teachings and events in the remainder of the Gospel.

When John 1:1 describes the Word as "a god," it establishes a quality of Jesus that helps us understand later verses such as John 1:18, and it teaches us Jesus' unique role in God's arrangement. It also provides a fulfillment to the Messianic prophecy at Isaiah 9:6 where he is called "Mighty God." At the same time, it strives to clarify that Jesus isn't the Almighty God by first saying that Jesus was "with God."

John 1:18 says in the NWT:

No man has seen God at any time; the only-begotten god who is at the Father’s side is the one who has explained Him.

In the Study Edition of the NWT, a note on "the only-begotten god" explains how important this designation is and how John 1:1 sets the stage for it.

the only-begotten god: John is here referring to the Word, “Jesus Christ,” whom he earlier calls “a god.” (Joh 1:1, 17) John speaks of Jesus as being the only-begotten Son of God. (Joh 1:14; 3:16) In this passage, John calls Jesus “the only-begotten god,” a term that emphasizes Jesus’ unique position in God’s arrangement. Jesus can rightly be called “a god” because of the way the term “god” is used in the Bible. This title conveys the basic idea of a mighty one, and it is even used of humans in the Scriptures. (Ps 82:6; see study notes on Joh 1:1; 10:34.) Jesus is “a god,” or a mighty one, because he is given power and authority from the almighty God, the Father. (Mt 28:18; 1Co 8:6; Heb 1:2) Because Jesus is the only one directly created by God and the only one through whom all things “came into existence” (Joh 1:3), he is appropriately called “the only-begotten god.” This expression shows that Jesus holds a unique position of glory and preeminence in relation to all of God’s spirit sons. As reflected in some Bible translations, some manuscripts read “the only-begotten Son.” But the earliest and most authoritative manuscripts read “the only-begotten god” (with the definite article in Greek) or “only-begotten god” (without the definite article in Greek).

If John 1:1c was omitted from John 1:1, it would not only be in violation of God's will for what should be conveyed, but it would also leave the reader unprepared to understand Jesus' preeminent position above all other creatures. I have never found a publication by Jehovah's Witnesses which refers to him simply as "an angel" because it fails to communicate the amount of glory, power, and honor that distinguishes God's Son from any other angel.

All Jehovah's Witnesses and the vast majority of Christian believers in the world agree that (as I said in point B of my removed answer), "To call an angel ‘God’ is blasphemy, and all Christians know that." I agree with the way Jehovah's Witnesses have ensured that their Bible, the New World Translation, never calls any angel 'God'. Please note the significance of the capital 'G' here. It's not a typographical mistake. We are speaking (in agreement) of no created angel ever being called 'God' with a capital 'G'.

This is hugely important with regard to answering this question, because it accounts for why the Jehovah's Witnesses, who believe that Jesus was created as the Archangel, translate John 1:1 as Jesus, "the Word was a god" - with a small 'g'. The question itself made the point that Jehovah's Witnesses believe the Bible shows Jesus to be a created angel, the Archangel. This ensures that the Jehovah's Witnesses do NOT have that blasphemy in their NWT! Good! That is something we agree on! If they had translated 'was God', they would have been calling this angel 'God'! But they do not.

So, let us be clear that - because they have this doctrine that Jesus is the created Archangel - they CANNOT translate the clause in question as 'was God'. They MUST translate it as 'was a god' to avoid this theological 'trap' of elevating a created angel to the rank of God.

The other reason given in the question had to do with the rule of Greek grammar known as Colwell's rule, with which the Jehovah's Witnesses disagree. Because they disagree with Colwell's rule, they feel free to translate 'was a god'.

The final part of the question (at the end of the questioner's comments) was what the Jehovah's Witnesses think the apostle John was trying to convey by writing, καὶ Θεὸς ἦν ὁ λόγος. In my deleted answer I included points about that from the Jehovah's Witnesses own Kingdom Interlinear of the Christians Greek Scriptures (1985, page 1139). I will say no more because it appears that I will then run the risk of having this second answer of mine deleted too. The point that seems to offend is what John wrote at the end of his Gospel about doubting Thomas but if I am mistaken, please advise me. However, that point directly bears on the answer to the last part of the question, which cannot be ignored because the apostle John wrote, in Greek, that Thomas called the risen Christ 'the God of me' and the grammar demands a capital 'G' just as it does at the start of John 1:1 when describing the God that the Word was with.

This clarifies the apparent misunderstanding of taking offense at the generally accepted Christian view that to call a created angel 'God' is 'blasphemous'. The Jehovah's Witnesses agree that that would be blasphemy, and they have avoided such a charge by ensuring their created Archangel (the Word who became Jesus) was merely 'a god' and NOT 'God'. This is a major reason why they think the apostle John wrote what he wrote without himself violating the first-century belief of Christians that no angel could be called 'God'. This is also a major reason why my first answer should be reinstated because this point vindicates the Jehovah's Witness rendition even though others disagree with it for other reasons. As far as the Jehovah’s Witnesses are concerned, they are maintaining consistency in their doctrine of the Word being the Archangel.