What purpose do angels serve (Protestant)? If God is omnipotent and can help His children in any situation and in any place, why does He have angels do His will? As I recall, angels are spirit like God.
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.