This doesn't just apply to people who troll for reactions.
There are topics that I just don't bother to discuss with family and friends, mainly sensitive topics like religion and politics. I have no desire to start an argument with my mother about whether there's a God. A discussion forum though, is there primarily for people to have discussions. I like coming places like this to talk about topics I probably wouldn't get the chance to discuss in real life without risking upsetting others or just creating awkwardness.
Although yes, I think the internet gives people a chance to say things they wouldn't say face-to-face, that's not always a bad thing. It gives people who for one reason or another won't or can't talk about certain topics a place to not only talk about them but get points of view from around the world. I don't think I'd know as much as I do about debate without the internet to practice on!
Some people will always abuse anonymity. That's just how it is, give a kid a mask and suddenly they're invincible. I don't think there's really any question that those people exist, but they're not the majority, and that's the important thing.