Personally, I don't know how you guys can keep your sanity discussing the various topics with people like faith. I try to avoid getting into such discussion nowadays. I don't want to have another mental breakdown.
Because it is a way to expose common and uncommon creationist concept to scientific review, to expose the shortfalls and invalid logic.
Because the purpose is not to convert Faith and the like, but to educate the lurkers reading the threads, people who may not have the scientific education but want to learn.
Because it is a way to learn from others in different fields.
Because it is entertainment -- as long as you don't take it too seriously.