A few reasons.
- I get free on-the-job training in written debate, research and effective writing in general, with rapid feedback if I do badly.
- I'm relatively new to atheism, and I'm crash-testing it. I'm actively seeking the best arguments against it to see whether I've missed something obvious. Nothing so far.
- Many religious folks paint a really horrid picture of atheism and atheists themselves. That reflects badly on me and any of my friends and family who are atheists. I want anyone who's genuinely curious about atheism to be able to ask an atheist about it directly, and set the record straight.
- You call this conforming to morals and customs? I spend half the time here explaining to people why I don't conform to their doctrines.