What is the purpose of angels? 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.