The people who do this, already know they're attractive. They just want to hear it because they're not getting enough attention in real life. Some even have boyfriends and girlfriends, but still ask. It's pitiful, isn't it?
I have a uploaded pic so people know what I look like. I like seeing others peoples pics too. Why? Because then I at least get some idea of who I'm talking to which is interesting to me. Same reason as why when I go out I don't wear a plastic bag over my head. I want to give people the opportunity to see who I am and if someone does want to judge me then why do I care? It's the internet?