I use avatars to remember who is talking so that I can get a better perspective on why the person posts what they post. If the person changes their avatar every week then I just remember their signatures. If the person uses neither than I have no idea who you are. Might as well be a ghost to me. (Sometimes I remember usernames....sometimes.)
As for me I tend to have an avatar for whatever I am currently obsessed with. Right now I am not obsessed with anything so I just use my neutral Snivy avatar.