I believe that you can't be down-voted, and therefore lose your hard-won points, by commenting. But if you answer, you can be down-voted. So for some participants, commenting is prudent.
And then there are the poor souls who have a useful comment to make, but can't because you need reputation points to make comments. So they leave an answer instead, which is completely inappropriate and just leads to more discouragement.
About that, there seems to be a recent trend that answer quality is so low that the only actual useful information is on the comments.