1. Members fear for the security of their jobs if their faces are revealed.
and/or
2. Members feel that their faces are not that photogenic or that they might not be considered that "good looking" by other members.
For me there are just three reasons:
1. Anonymity, which covers #1, above, and many more of the responses to this thread.
2. This site isn't about big faces, it's about big dicks. Besides, I've had a lot of gals and guys tell me that I'm really good looking, even studly. I've also had a number of people tell me that I'm ugly. But I've never had anyone who's had first hand experience with my cock say that it's ugly. So I guess that kinda covers #2, above, doesn't it?
3. Security, and this is very much associated with my #1 reason. You never know when someone who is just a cruel misanthrope, or whom you pissed off on one of the threads, might just gather up all your photos and "out" you to the world. I'm pretty open about what and who I am but there are a lot of people in the general public and especially in my immediate family that I wouldn't want exposed to my activities here.