I definitely think that the avatar gives a look at a persons personality, either through a glimpse of something that they like or enjoy, or in a more indirect manner by seeing something that they want to project. Ok, sure that might be reading too much into some, if not most, of them but it is fun to think about sometimes
In most cases it can be the avatar that makes me click on their name to view their profile.
For my own I wanted to give people an idea of who I am and what I like.
Dave