Role specific maia

[Earendilyon]

I don't think they got specific tasks asigned to them by the Valar or by Eru. They were offspring of Eru's thoughts and they were all the offspring of some other thought, I guess. They had their own inclinations and characters. Just as humans can more love the woods than the seas, there were Maiar who loved one thing more than another thing.

Also, the Istari were not from the beginning asigned to become Istari in the Third Age of the Sun. They were Ainur and had their different things to do and places where they were most. For example, Saruman was an Ainu interested in things as forgery and smitying and such, whereas Gandalf was an Ainu who often gave solace and dreams to the Valinorean Elves.