Here's a personal answer that may help:
Religion has affected every stage of my life, and continues to. I am an atheist, but am surrounded by religion. It can and has had an impact on my friendships, romantic relationships, familial relationships, and worldview. Some has been positive, some negative. But it's always there.
At one point, I wandered into this forum here on KMC (this was, hell, probably 2006/2007). I was experiencing a transition in my beliefs, and it occurred over a period of years. I'm very methodical, so it included a lot of research, as well as a lot of discussion and debate. And KMC was great for that. Sometimes I agreed with people. Sometimes I disagreed. Sometimes I thought I was legit talking to someone with a mental illness. Other times I discovered new ideas or new ways to look at things. It was cool. KMC isn't a hotbed of all religious thought, certainly, but it was a diverse cross-section at one point, and was supplemented by research from numerous other sources.
This discussion and debate helped me refine my own thoughts on religion. And for discovering how people would react to my opinions. It was amazingly helpful in that respect. So, even as an atheist, some of the most interesting and useful threads to me were Christian ones, because that's the prevailing culture I continue to be surrounded by. And because part of my process was exposing myself to a variety of dissenting opinions, because I didn't want to shelter myself from opinions and ideas that differed from my own.
Any time I hop in now is a vestige of that time. I don't actively engage in many debates these days. Debate religion for a decade and you start to see a lot of the same arguments. But occasionally you get the itch, and it's fun to engage someone, even someone you vehemently disagree with.
So that's why. For me, at least. With possible rare exceptions for the truly onerous that I've encountered here, it's never been about shouting someone down about how they're wrong. But it has, on numerous occasions, been about explaining my position and how it disagrees with very fundamental assertions made by others, which some people take as a personal affront. It's often a tricky line to walk - where does an attack on an idea end and an attack on a person begin? To the deeply religious, it's often the same thing, so it's hard to disagree without eliciting a nasty reaction. But I always at least tried to stay on the side of intellectual - not emotional - debate. And, on the whole, it's been a stimulating experience.
Hopefully that explains my perspective, and provides one possible answer to your question.