Back in the real world, it's much harder to have a reasonable debate.
It's actually pretty difficult to debate in person. You have to remember all your facts, you have to articulate clearly, and you have to work very hard to keep it civil. A conversation about religion can turn into fisticuffs with one wrong choice of words, or at least become highly awkward. Multiply that by 1000 if there's an audience. Back when I was on my college debate team, the issue of the day was teaching evolution...not only do you have to have to be ready to counter whatever Bible quote or factoid they can throw at you (pro-tip: if you ever want to debate a creationist, read and study the Bible. Seriously. You might be an atheist, but it's to your benefit to be able to appeal to religion: I've had more success by suggesting that "Maybe god created a universe of scientific laws" than with any discussion of scientific methods and facts. Winning a debate means finding middle ground.), but you've got a theater with 500 people in it, half of whom look like they're ready to rip your head off, and you'll have to be as civil and polite as possible because a key creationist tactic is to represent science as part of a secular agenda out to destroy religion and they will immediately pounce on any opportunity to make you look like the bad guy.
On the internet, you don't have to respond immediately. It takes a lot of the stress out of being on the spot. Instead of fumbling for a fact I had prepared to make a case, I can Google it and have all the time I need to work it into a rebuttal or argument. You can think about what you're going to say.
Anonymity is important. Offline, there are things that might be inappropriate to talk about. In a conservative setting, you can get yourself in a lot of trouble by opening a debate about abortion, for instance, regardless of where you stand on the issue.
There are things you can debate on the internet that you cannot debate elsewhere because it's easier to find participants. Take the Escapist, for instance. Being a video game website, politics surrounding video games and the Internet are very important to most of us, anywhere else people who have opinions on net neutrality, free-to-play, microtransactions, which equipment or platforms or games to buy, and DRM are in the minority. Here, you can easily find people to talk about that stuff with.