Because people are addressing questions or remarks to atheists there, and they wish to respond?
Because people are saying factually incorrect things there, and they wish to clear up misconceptions?
Because they wish to defend people (atheist or otherwise) who are being insulted, belittled, or harassed for not being the same religion as the person doing the insulting or whatever?
Because they're bored, and think it's fun to watch religious people argue, even if they don't stir up any drama themselves?
Because they find the subject of religion abstractly fascinating, even though they are not adherents of any religion?
And probably a lot of other reasons.