I wanted to say that as a comment (again) on Olin's answer, but it's actually too long to fit.
Technically, that is right, there are questions, answers, and that's all. This is if we consider, as Stack Exchange mainly encourages us, that this site should be a repository of great answers on great questions. Amen.
But there are also people. People with problems they'd like to solve. And that we can help. And sometimes it even takes several people to help one. Look at this question for example: How can a USB-RS232 converter manually assert its TX pin?
That is an extreme case: 19 comments and not a single answer. Why? Because nobody had the full answer. People were just adding their brick to the wall, and at the end, the OP made progress towards an appropriate solution (even you, Olin, made a comment - well, I have seen a lot of comments from you that were more helpful than this one, however). So, thanks everyone, we helped a guy. It does not really fit with Stack Exchange's expectation of what questions and answers should be, but we did help a guy, and that counts.
Note that here, nobody cared about rep (either to gain it or fearing to loose it). And it's actually almost always the case. I think nobody really put a comment instead of an answer because they fear loosing reputation. This is what I said on the top, but it was obviously a joke. Anyway, answers that get too many downvotes can be deleted and you get the rep back, right? However, I may comment because I fear being wrong and mislead the OP. That's different, and rep has nothing to do in it: I know that comments should be taken with caution, and I expect people to do the same. And when I don't have time to check what I say, I put something in a comment, eventually with some warning inside, so the guy who reads it know that it may be a solution, but maybe not.
And this way, I'm actually in line with SE policy: I write a full answer only if I feel it is great enough (well, I may have written not-so-good ones also, but that wasn't my intent). If I had made a full answer instead of each comment-answer I have written, this site would be less clean. But some of these comment-answers, I believe, have been helpful.
So, to sum up:
- I comment because I fear leading the OP in an approximative direction, but I still feel what I say may be useful (and I put the appropriate warnings in the comments). Recursive example
- Or because I know only a part of the solution, not the whole thing.
- Or for the reasons The Photon and PlasmaHH already pointed out.