What purpose do angels serve (Protestant)?

I do not want to repeat @Lesley's excellent answer, but add to it.

• Angels in scripture are always described as male.

• Angels cannot reproduce (Matt.22:30). (Those who dispute this are in the minority.)

• Two angels guard the tree of life, preventing humans from entering the Garden of Eden in our sinful state, according to Genesis 3.

• The "angel of death", called the "destroyer" in Exodus 12:23, is sent to execute the judgment against the firstborn of Egypt.

• Another angel accompanied Israel for guidance, protection and to fight for them:

20 “See, I am sending an angel ahead of you to guard you along the way and to bring you to the place I have prepared. 21 Pay attention to him and listen to what he says. Do not rebel against him; he will not forgive your rebellion, since my Name is in him. 22 If you listen carefully to what he says and do all that I say, I will be an enemy to your enemies and will oppose those who oppose you. 23 My angel will go ahead of you and bring you into the land of the Amorites, Hittites, Perizzites, Canaanites, Hivites and Jebusites, and I will wipe them out. (Exodus 23:20-23)

• In every instance that I can find where God was not sending an angel to execute judgment, the angel said "Do not be afraid", so reassuring believers of God's benevolent intentions is also a purpose of sending angels.

• In the story of Sodom and Gomorrah in Genesis 19, the angels were sent to perform an investigation of the behavior of the residents of Sodom, Gomorrah and the neighboring cities.

• In Jude 1:9, there is the peculiar task of guarding the body of Moses to prevent Satan from making use of it for sinister purposes.

• When I studied the Book of Job, I found mention of thirteen means whereby humans and God communicate. One of those means was angelic mediation. In Job 33, Elihu says this about angelic mediation, to teach holiness to people:

  23 Yet if there is an angel at their side,
  a messenger, one out of a thousand,
  sent to tell them how to be upright, 24 and he is gracious to that person and says to God,
  ‘Spare them from going down to the pit;
  I have found a ransom for them— 25 let their flesh be renewed like a child’s;
  let them be restored as in the days of their youth’— 26 then that person can pray to God and find favor with him,
  they will see God’s face and shout for joy;
  he will restore them to full well-being. 27 And they will go to others and say,
  ‘I have sinned, I have perverted what is right,
  but I did not get what I deserved. 28 God has delivered me from going down to the pit,
  and I shall live to enjoy the light of life.’ (Job 33:23-28)
  

This article discusses Protestant reformer John Calvin's teachings on angels, as well as other contributions from the Reformed perpective: https://www.reformedreflections.ca/series/angels5.pdf

One quote from Calvin:

"The whole praise of salvation rests with God alone; for the angels do not bring help to whomever they wish to help, nor are they moved by their own will; they only obey the rule of God."

The article asserts that Calvin and other reformers deemphasized angels, in comparison to their prominent place in prior Medieval thought. They acknowledged their existence and function, but did not devote much space in their writings to angels.

Another article speaks about the Baptist perspective:

http://www.sbclife.net/article/74/angels

"Angels are always subordinate to and carry out the will of God; they do not act on independent initiative," writes Millard Erickson in his text, Christian Theology.

• Putting angels in such perspective (of not being over-enamored with them) seems to reflect an attitude derived from Hebrews chapter 1, which describes them in contrast to Jesus, the Son.

• Also, Angels are completely at Jesus disposal should he call for them, according to Matthew 26:53.

• The SBC Life article also rejects the existence of a personal guardian angel assigned to each person as an ancient Jewish tradition as well as a Roman tradition, so not rooted in Scripture. However, others cite Acts 12, where an angel released Peter from prison and surprised his friends:

  13 Peter knocked at the outer entrance, and a servant named Rhoda came to answer the door. 14 When she recognized Peter’s voice, she was so overjoyed she ran back without opening it and exclaimed, “Peter is at the door!”

  15 “You’re out of your mind,” they told her. When she kept insisting that it was so, they said, “It must be his angel.”

• This article cites John Calvin as being undecided on the question of personal guardian angels:

https://gentlereformation.com/2014/11/10/guardian-angels/

• This article quotes Jonathan Edwards extensively on the subject of angels.

https://www.apuritansmind.com/puritan-favorites/jonathan-edwards/miscellaneous-writings/angels/

From a Protestant perspective, based on what the Bible says:

It is true that angels are spirit creatures, but unlike God, they are created. Although angels were created as an order of creatures higher than humans, unlike humans, they are not created in the image of God (Genesis 1:26). Unlike humans, angels do not have to study the past because they have experienced it. Therefore, they know how others have acted and reacted in situations and can predict with a greater degree of accuracy how we may act in similar circumstances. However, their knowledge is limited. For example it says in Matthew 24:36:

But concerning that day and hour no one knows, not even the angels of heaven, nor the Son, but the Father only.

One purpose of angels is explained to us in Hebrews 1:14:

Are they not all ministering spirits sent out to serve for the sake of those who are to inherit salvation?

The Bible describes many different functions that angels perform as they serve God. They praise God (Psalm 148:1-2; Isaiah 6:3). They worship God (Hebrews 1:6; Revelation 5:8-13). They rejoice in what God does (Job 38:6-7). They serve God (Psalm 103:20; Revelation 22:9). They appear before God (Job 1:6; 2:1). They are instruments of God's judgments (Revelation 7:1; 8:2). They bring answers to prayer (Acts 12:5-10). They aid in winning people to Christ (Acts 8:26; 10:3). They observe Christian order, work, and suffering (1 Corinthians 4:9; 11:10; Ephesians 3:10; 1 Peter 1:12). They encourage in times of danger (Acts 27:23-24). They care for the righteous at the time of death (Luke 16:22).

Some angels are designated as "cherubim," which are living creatures who defend God's holiness from any defilement of sin (Genesis 3:24; Exodus 25:18, 20). "Seraphim" are another class of angels, mentioned only once in Scripture in Isaiah 6:2-7, and are described as having three pairs of wings. They apparently have the function of praising God, being God's messengers to earth, and are especially concerned with the holiness of God.

Yes, angels serve God by doing His will, and it is God’s will that his holy angels serve believers. It is also God’s Will that his holy angels will return, along with Christ Jesus, to judge the world prior to His Millennial Reign.