A: Imagine moving to a new city where everyone believes that people with blond hair are criminals, and you have blond hair. Even though you're pretty sure the people in this city are wrong, you'd better be prepared to educate yourself about the issue, because you're going to have to deal with it whether you want to or not.
That's the position most atheists find themselves in. Our culture -- American culture, particularly -- is steeped in religion. Our population is overwhelmingly Christian, overwhelmingly church-going, and overwhelmingly distrusts atheists:
From a telephone sampling of more than 2,000 households, university researchers found that Americans rate atheists below Muslims, recent immigrants, gays and lesbians and other minority groups in “sharing their vision of American society.” Atheists are also the minority group most Americans are least willing to allow their children to marry.
Given that kind of hostile environment, many atheists feel they have to be well-armed with knowledge in case they're confronted by an overzealous theist.
Other atheists simply find the subject of religion interesting, much like a literature enthusiast loves studying fiction even though they don't really believe the events in them are true.
Still other atheists were raised in a harsh or hostile religious environment and have bad feelings towards religion as a result. They have a rich knowledge of their denomination by virtue of their upbringing and thus are more likely to engage in a debate about religion than someone with no such information.
Finally, some atheists just like a good argument, and nothing gets people to argue faster than religion.
And as always, The Universal Caveat applies -- you're likely to get as many answers to this question as atheists you might ask!