Since I basically was the one who started this, I might as well elaborate on my reasoning.
First of all, the reason why we put questions on hold (which results in them changing status to "closed" if not reopened, but technically there is very little difference between "on hold" and "closed" and for a long time questions were "closed" immediately) in the case of questions which are on topic but simply poorly done, is so that we can figure out exactly what the asker is looking for without the question being a moving target for people trying to answer it.
So much for background. Now, why is it a bad thing to answer a question that is close-worthy? For one, as has already been said, it encourages behavior we do not want to see. We want to provide specific answers to clear, specific questions. We have had lots of discussion previously both on Meta and in chat as to what makes a good, answerable question in our case, particularly since worldbuilding questions have a tendency to actually be rather broad. Those discussions have more or less resulted in a site consensus for what is narrow enough to be answerable, and what is too broad to be answerable. This naturally results in some questions falling to the "not answerable" side of the spectrum. Because newcomers tend to not be familiar with how the Stack Exchange system works, and be familiar with each site's specific guidelines, this happens more often with questions from low-rep users. That is normal and expected. (We shouldn't lower our standards just because a question is from a new user; however, to be able to fix the problems with their question, they may very well need more guidance than an experienced member of the community would.)
See the difference here? Only if an answer actually answers the question should it be posted as an answer. If an answer simply requests more information, then it should be posted as a comment. The difficult part comes when an answer does both. At that point, it becomes a judgement call whether the answer is primarily an answer (however incomplete it may perhaps be) or if its primary purpose is to somehow request clarification from the person asking the original question. (Rule of thumb: if you even think about clicking "close" or "flag", or starting your answer with something like "you really should clarify X, Y and Z, but...", then the question is likely not good enough to be properly answerable.)
The problem is when someone comes across a question that they feel needs clarification and vote to close for that reason, yet feel the question is clear enough that an actual, possibly incomplete, answer can be written. The two actions are mutually exclusive! A question either should be put on hold because (a) it is a poor fit for the site's subject scope, or (b) it cannot reasonably be answered in its current form; or it is a candidate for answering by someone with the subject matter expertise needed. A single question cannot simultaneously and in a single person's judgement be both answerable and not answerable!
By answering the question, you imply that in your opinion the question is clear enough that an answer can be given. But by participating in closing the question, you deprive others of even the possibility of posting answers of their own until the question is reopened -- which, depending on circumstances, may or may not happen!
Also, by answering the question, you give the OP what they are after (even if only partially), which as has already been pointed out reduces the likelihood that the question will be revised. By putting the question on hold without answering, we basically force the OP to clarify the question if they want answers (which is usually, but not always, why people ask questions in the first place). We also post comments describing what's wrong with the question and ideally how the OP can fix that so that answers can be given.
Further, editing a question that has answers requires much greater care than if the question does not yet have any answers. While not spelled out in the terms of service or anything similar, it is considered good form to ensure that any edits made to a question that has answers will not invalidate any existing, previously valid answers. Note that significantly deviating from the intent of a post's author is a reason to reject a suggested edit; while the owner of a post can always edit the post, this remains a good guideline even when the edit is being made by the OP.
Putting the question on hold ensures that no answers are added while the quirks of the question are worked out and fixed. This gives the community greater latitude in suggesting how the OP may improve the question, including narrowing the scope of the question in ways that would come with a high risk of invalidating existing answers if there were any. This saves time and grief for everybody involved.