There really is no "direct" benefit to having an avatar. It is just neat to be able to add that little bit of personalization.

It's like a company offering free soda. There is no benefit, if anything it is harmful and the company would be better off sticking to the drinking fountains. But some companies do it as a reward to their staff. I just think of avatars as that extra little reward to forum members I guess.

Over at DevShed you have to be a member for X long with X amount of posts before getting an avatar. It's I guess neat to be able to say, "I've been a worthwhile member and can display my achievement proudly".

So, like I said, I can't prove that it is beneficial - except as being a milestone.