It's an Internet forum/blog thing. I guess it makes one's Internet experience more authentic.
Beresford does a good job summing it up, and makes a good point about the fact that almost no one uses his real name here. I think the forums sometimes lull us into a false sense of security/e-friendship. This often pops up in buying and selling transactions elsewhere -- "I thought I could trust him because ... " But the bottom line is that anyone can access the forums, and, as has been pointed out, information is uncontainable on the Internet.
Would you invite a random AAer into your home? I wouldn't dream of it. So why would I offer up my picture and, in essence, trust every single person who comes along with my identity? Props to those brave souls who feel comfortable doing so, but I'm not one of them.