First of all, it is an easy Way to keep Track of One's Posts.
Besides, yes it does change a Person a lil. People with very "serious Avas" *sound* quite a Bit more serious than People with more uh..."open" Avatars.
Besides, Avas make a good Conversation-Starter. You can leave Someone a Comment saying "Hey, I like your Avatar" and slowly start a nice Conversation, for Example. Instead of just going "Well...hi". =p
Also you can kinda spot some similar Interests sometimes, just by looking at them. That's cool.
Personally, I normally pick my Avas according to my "Mood" for a longer Time-Span. For Example during October I had a halloweenish Ava on a Forum. On some Forums I pick one Ava "forever". I mostly pick pretty *wisely*, so no Need to constantly change. And on my MSN, which I access every Day, I have "Mood-Avas" which always show my current Mood. I made them for me. I can express any Mood I am going thru. Mostly I did that for People to see when they better back off etc.

But that is really the only Place where I change my Ava so often. On normal Forums I just change occasionally, when a new "Era" begins. And most likely I won't change my Ava here. Also for identifying Purposes for Others. I'd rather change my Profile Pic sometimes, maybe. *shrugs*