Aside from them being generically created "...for God..." like everything is, does the bible give a set of duties or purposes that angels have?
Per John McArthur in "The Glory of Heaven":
Whenever angels do appear to people in Scripture, it is in the role of a messenger. In fact, “messenger” is the primary meaning of the Greek word angelos. So the angels provide a sort of heavenly messenger service, and we get glimpses of this throughout Scripture. The angel who appeared to Mary at the Annunciation identified himself: “I am Gabriel, that stand in the presence of God; and am sent to speak unto thee” (Luke 1:19). On this particular occasion, perhaps because of the sacred importance of the message, the highest-ranking archangel from the very presence of God was sent to Mary to deliver the word.
If you think about Angels - be it the visitors to Abraham, the visitors to Samson's parents, even to Mary and Zechariah, their job function is obvious. They have a message to deliver - "You're going to have a son' or 'God is going to destroy this city' or 'The Lord has heard your prayer,' etc. That these 'messengers' are being couriers is obvious from the name. Angelos means messenger in Greek. To quote another post, Angelos is a job not a species.
The closest thing you have to anything other than simple message would be the incident in Daniel 10. There, Daniel recieves a message from an Angel, but the Angel was delayed for three weeks. He says:
12 Then he said to me, “Fear not, Daniel, for from the first day that you set your heart to understand and humbled yourself before your God, your words have been heard, and I have come because of your words. 13 The prince of the kingdom of Persia withstood me twenty-one days, but Michael, one of the chief princes, came to help me, for I was left there with the kings of Persia, 14 and came to make you understand what is to happen to your people in the latter days. For the vision is for days yet to come.”
Here, Michael had to help "fight" the demonic presence of the PRince of Persia. But again, why? So that the Angel could deliver the message.
Angels have several duties:
Protection for His children:
Psalm 91:11 - 12: For he shall give his angels charge over thee, to keep thee in all thy ways. They shall bear thee up in their hands, lest thou dash thy foot against a stone.
To minister to His children:
Hebrews 1:14: Are they not all ministering spirits, sent forth to minister for them who shall be heirs of salvation?
Revelation 22:8 - 9: And I John saw these things, and heard them. And when I had heard and seen, I fell down to worship before the feet of the angel which shewed me these things. Then saith he unto me, See thou do it not: for I am thy fellowservant, and of thy brethren the prophets, and of them which keep the sayings of this book: worship God.
Exodus 23:20: Behold, I send an Angel before thee, to keep thee in the way, and to bring thee into the place which I have prepared.
To hold back destruction:
Revelation 7:1 - 3: And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree. And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea, Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.
- These four angels seems to be special, they hold back destruction as portrayed by the winds
The angels move the throne of God:
Psalms 18:10: And he rode upon a cherub, and did fly: yea, he did fly upon the wings of the wind.
Ezekiel 10:1 - 2: Then I looked, and, behold, in the firmament that was above the head of the cherubims there appeared over them as it were a sapphire stone, as the appearance of the likeness of a throne. And he spake unto the man clothed with linen, and said, Go in between the wheels, even under the cherub, and fill thine hand with coals of fire from between the cherubims, and scatter them over the city. And he went in in my sight.
Messengers: Well there are so many examples. Just for the sake of mentioning: The messages to Moses, Joshua, Gideon, Manoah, Manoah's wife, Zechariah, Mary, Joseph, etc...
Special role in Prophecy:
Revelation 1:1: The Revelation of Jesus Christ, which God gave unto him, to shew unto his servants things which must shortly come to pass; and he sent and signified it by his angel unto his servant John: Daniel 10:21: But I will shew thee that which is noted in the scripture of truth: and there is none that holdeth with me in these things, but Michael your prince.
The Angels Gabriel and Michael seem to have a special role as only Daniel, Gabriel and Michael are "holding these things".