The "reason" for closing such questions is that they are poor questions. Questions should be judged on their own merits, and not on the basis of the quality of answers which they may have attracted. That being said, terrible questions (like the one you cite) can sometimes attract high quality, well thought out answers. I concede that such questions should probably be dealt with in a more nuanced manner. However, I do not think that there is a clear consensus on this issue, so let me take a stand (upvote to agree, downvote to disagree):
Low quality questions with higher quality answers should be closed, but should not be deleted.
Simply Beautiful Art made (in my opinion) a very good case for this course of action. There are (essentially) three parties who are potentially affected by the closure of a question:
- The original asker, who is not harmed because they have an answer to their question.
- The people who have donated their time to provide answers, who are not harmed because their answers are still visible.
- The community at large (i.e. other users who might someday look for an answer to the same question), who are not harmed because closed questions are not removed from the site (unless they are highly downvoted and have no answers with positive scores).
Thus no harm is done by closing a low quality question with high quality answers. On the other hand, I do think that harm is prevented by this course of action, as it sends the signal that such questions really shouldn't be asked, and it prevents Johnny-come-latelies from providing further answers.