I assume that the OP's asking 'If the question is not helpful and clear, then why answer?' was intended as a rhetorical question, but there are, in fact, situations in which one may reasonably want to do that. In particular, if the question already has answers, and one thinks that these answers are wrong, and it seems unlikely that the question will be deleted, one may be concerned that future visitors to the page will be led astray by these wrong answers. One may in such a situation be eager to prevent that by posting what one believes to be the correct answer, even though one thinks that the question is trivial, uninteresting, badly formulated, etc., and so not deserving of an upvote.
The desire to prevent future visitors from being led astray by bad answers is so entirely independent of one's judgement of the question itself, that there would be no contradiction even between downvoting a question and answering it. In fact, it could be reasonable for somebody to vote to close a question before any answers are posted, in the hope that the question will disappear from the site, but then later, after it turns out that one's vote was unsuccessful and that the question is here to stay, want to make sure, for the benefit of future visitors to the page, that it gets (what one regards as) the best answer possible. (The preceding sentence is also a partial answer to the recently posed question about whether there is an inconsistency between voting to close a question and answer it.)
This answer is intended as a supplement (rather than an alternative) to the one already posted by Mr. Leach.