The title says all.
For example, this question has two answers, one accepted. I don't know exactly how this happend, but now what's the point to keep it "On Hold"?
I notice one of the more obvious reasons is missing from the answer list.
Marking such a question as "On Hold" indicates that (a sufficient part of) the community finds the question inappropriate as is.
This is a critical information if we have any desire at all for users of the site to be aware of what the community finds inappropriate. Leaving such questions open -- or especially reopening them -- sends the diametrically opposed message: that the community accepts such questions, even if the downvotes show that it doesn't like them.
EDIT: This answer is in response to the original question of whether having an answer obviates reasons why a question should be put on hold. Arguments about whether there is ever a reason to put a question on hold are beyond the scope of this answer, and IMO, are quite off-topic for this meta thread.
This will not necessarily answer your question, however ...
I was the first one to vote to close the question. I noticed the question and noticed that this was a statement-only type of question. I then posted a comment asking for the OP's own thoughts on the problem. After I did that rschwieb posted what I considered a better(nicer?) comment also asking the OP to include "the partial work that you've done so far". I then deleted my comment because I thought rschwieb's comment said it better.
At the time of my close vote there were no answers. From what I remember I also believe that the second close vote came before any answer had been given. The OP then at some point did include more to the question. But the added was simply: "I have tried for a whole day but still no way to compute it out. Thanks a lot". This hardly counts as providing the partial work that the OP has been doing. This, to me, suggests that maybe the OP really just wants us to do his/her homework. Or it at least shows that the OP does not want to provide the partial work. So, I think a closing is appropriate.
Also, I don't see the existence of an answer as proof that the question is appropriate. If it was, then one could force a question to remain open by simply posting an answer. The point with closing a question isn't just that no answer can be given, but that (for whatever reason) the question doesn't fit/isn't appropriate for this site. It is my opinion that closing a question for no effort can be done whether or not there is an answer.
Also, if enough people in the community want to reopen the question, then I suggest that they cast a reopen vote and submit a post in the reopen-Meta-thread.
I notice that the OP has several questions that show no effort. And I notice only one other question that has been closed for no-effort. With several questions and even an answer from this OP, this person seems interested in participating and contributing to this site. Maybe it would be good if this person would begin to show work in questions, and I hope that this closing will help that.
What's the point to put/keep “On Hold” a question that has already an accepted answer?
Reason #1: to push the OP toward automatic question ban. Since the question ban formula is not disclosed, it is not clear to me how heavily it weighs past questions being closed. The help center does not mention closed questions at all, but after consulting meta.SO I think this may be an oversight.
Reason #2: to push the question toward deletion. Questions have to be closed before they can be deleted. Sure, the deletion does not happen often, and when it does, there may be collateral damage (deleted answers). That said, the answers to the kind of questions we are talking about are rarely worth much.
War is hell.
Closing answered questions (as off-topic) disrupts the close-as-duplicate function of the site if a similar question is posted again.
If a new question duplicates the closed one, closing the new question prevents any new answers, which violates the premise of duplicate closing, that there will be one place to gather answers and add new answers.
In the case where "low effort" question X gets basic answers and is closed, and is duplicated by "high effort" later question Y (that on its own would have received more attention), closing can create situations where the poster of question Y is in some sense punished for [five close voters' opinion on] past actions of poster X.
Not closing the later question leads to duplication of effort, and creates a special "punitive closing" (or whatever the closers think they are doing) exception to override the ordinary and useful functions of marking duplicates and of doing so in a consistent way across the site.
Posting a link with no close vote at the new question increases the chance it will go unanswered, and leaves potential answerers (of the new question) in a state of uncertainty about whether the question will be kept open long enough to be answered. Should they spend time thinking about a question they will not be able to post about? Risk typing in an answer when the close vote count is at 3 or 4?
A minor but interesting point is that closing answered questions from the Evil Homework Posters who are the target of closers' votes, can be expected to cause more acceptances on low-quality answers. The more astute EHPs understand that it is game theoretically advantageous to grease the wheels and always accept one of the answers. If the natural process of accumulating more answers is cut short, there will often only be the superficial answers to select from.