Why do some people post comments that are actually answers to the question? Wouldn't posting an answer be better?
Most often I do this when I have some input to give that I think is helpful, but I don't have the time/care to write a detailed answer that meets my personal quality standards. Rather than an incomplete answer or not helping at all, I'll leave a comment to point them in the right direction.
Sometimes I'll put in a half-answer in the comments when I'm really not sure if I'm right or not, hoping that somebody can confirm what I'm saying. Then I'll go off and research the answer, and by the time I get back somebody else has confirmed what I said in a real answer, so I'll vote them up, or if nobody has, I'll answer myself.
I'm really just trying to spur a quick correct answer, and not put down something as an answer if I'm not sure it's right.
If I vote to close a question, I refuse to answer it as well. But sometimes I will give some input in the form of a comment. Occasionally such a comment from me will be a real answer.
Alternatively I'll post "have you tried ...?" comments. Someone posts a question saying they want to know how to parse XML, so I'll say "Have you tried XSLT/XPath?" with a link to a resource explaining it. You could call that an answer, but from my point of view it's just a verbose link.
To me, it's an answer if I'm supplying real information that I generated on my own, or aggregated on my own. If it's just a link it's not an answer. Your opinions may differ, but that's how I see it.
Comments cannot be downvoted. For unclear questions, if the question is clarified or other people interpret the question differently then people might downvote an answer that does not answer their interpretation of the question. Also someone may be afraid of losing rep from an answer that may not be what people want to hear if the truth is unpopular or politically incorrect.
Hopfully avoiding downvotes is not politically incorrect or else I should have posted this as a comment. :(
There are probably as many answers to this question as there are people answering in comments...
I'll do it sometimes if I have a suggestion that doesn't really seem like a complete answer, if the length and formatting restrictions of a comment allow it.
Sometimes, I just don't want to bother writing up a good answer, so I'll use the comment to post a hint, in the hope that it'll prompt someone else to go to the trouble of researching and fleshing it out.
Others have noted that they'll use comments to provide answers to inappropriate questions. I'll usually still post an answer in that case, but mark it CW... however, this can upset some people, and I'm not entirely sure whether it works to encourage other CW answers (ok), or just encourage other answers (not ok). So I'll occasionally post my answer as a comment then as well.
One possibility is that someone wanted to answer a question which was closed while they were preparing the answer, so they posted a comment instead.
Occasionally it's not obvious when your response should be an answer or a comment. I have sometimes commented on questions, and it's turned into a discussion that results in an answer. YMMMV :-)
Because very often that "answer" isn't really an answer, but more like a hint or a suggestion. Which might be enough for some OPs, while not for others (and yeah, for some anything less than gif-me-teh-codez isn't).
Also, it's expected that answers are more profound and elaborated. For example one-liners are subject to deletion even if they actually answer the question.
If I vote to close a question because it's not appropriate for SO, etc., but it is a question that I know the answer to, I will often add the answer as a comment to be helpful but not encourage other answers.
Other times, like when my answer is really a joke and the question isn't CW, I'll choose to answer with a comment so that it isn't confused with a real answer to the question.
One reason that was made explicit on Chinese Stack Exchange is the following (emphasis added):
Just being allowed to comment already is a great favor. Besides some users are afraid their answers will be marked down, which means their score will go down. In fact there seem to be some powerful users who will miss no opportunity to take revenge on other users and mark their answers down. For this reason these users restrict themselves to writing comments.有用户认为，仅仅被允许撰写评论本身就是十分多么大的恩惠。 此外有用户害怕其答案很可能会受到扣分，导致得分会被减少的。好像有些力量大的用户总是伺机向别的用户报仇而使其答案受到扣分。因此后者就限制自己只是提供评论而不是任何答案。 – user6065 Oct 14 at 15:49
In other words, there appears to be infighting in some Stack Exchange communities, so people who are afraid of downvotes write comments instead of regular answers.