When a question is not entirely clear, some people will make an answer asking for clarification, some will use comments.
What solution is best for what situation?
If you're going to need more than 600 Characters to state your point, you're going to need an answer.
If you're wanting to give examples, you're going to need an answer.
If you're expecting feedback on your thoughts, you're very likely to want an answer.
If you have a question however, you don't want an answer. You want to:
Expand on the question. ( Preferred )
Comment on an answer. ( Only if its explicit to an answer, or possibly, a confusion with something said in a comment )
Comment on the question ( In the case the question is unclear )
If you have something to say that is comical, just for laughs, commentary, or otherwise adds no value to the answer/question ( Such as explaining why you voted up, voted down ), please, use a comment, not an answer.
If the question is not community wiki, and you want to community wiki your answer, think twice about it, usually that's an avenue to avoid downvotes, and if you're doing something just to avoid downvotes, then its not likely very valuable information, and may be more suited to a comment.
Low rep users can't comment on the following:
Questions they didn't ask
Answers on questions they didn't ask.
I don't like this much, but its how it is, and in those situations, answering with a question is the only way to provide feedback.
If you have information which is intended as a temporary solution until the answer is clarified, then putting that in an answer is acceptable. Putting some of your question in that answer is also ok. If you have information which may help with the asking of the question ( ie: giving scenarios ), then putting that in the answer might also be acceptable.
If it is a solution, or has a solution part, (that can be accepted) use an answer, for everything else use a comment.
An answer in isolation to all other answers and all comments should answer the question. The one possible exception to this is to refer to another answer for the sake of not duplicating it's content and then adding significant new content.
Everything else is a comment.
Use coments for clarification. That's why are there. Too many (non-)answers can force the question to community-wiki mode, which should not happen, if a clear answer to the question exists.
If you browse old questions, it could be the case, that people used answeres instead of comments, because comments were not implemented from the beginning.