It tends to lend credibility to the author. It also helps with memory of who you are and what you stand for. A lot of communication depends on non-verbal cues and facial expressions. Faces say more about the author than an icon that means something to you, but not to others. Communication depends on face to face interaction. Humans would rather communicate with a face than with a concept.
I think the photo you use to show the world can go a long way towards presenting who you are as an online persona. It is really only a small thing, but considering just how many people are out there on the web, every little distinction could help. All in all I think a good photo of yourself (I say yourself because I agree with Austinstar's opinion) can lend a little more light to the persona you are presenting. A small part of a bigger picture, if you will. For example: I use my pic because I like the way I seem to be deep in brooding thought which ties into some of my philosophical pieces of writing. I dont feel that I am not the same writer without said picture; but it does lend a little to the picture I am trying to paint.