My theory is that we have very few places where it is possible to see and understand the range of real (as opposed to totally porned up) human sexuality. This site allows a glimpse into the sex lives and interests of a large number of men, as well as an additional small number of women. It is an uninhibited discussion, fascinating even when it turns a bit moronic, as it often does.
There are additional virtues, including off-topic discussions and friendships that form, but I think the main attraction for me is that the site supplies a view into the heads of other people that is usually kept out of my sight. Plus, I get to see their dick pics...