Questions that happen to be unclear or lack focus are primarily closed to prevent users from posting answers that would unlikely be useful or that would be hard/impossible to rate or that would likely be wrong-footed by subsequent edits to the question (and, sure, as terdon says, also to signal the asker that some improvement is needed; though, in my opinion, leaving a comment would then be much better than voting to close).
It doesn't make much difference whether a vote to close is cast before or after a user has been told what is missing from their question. And the purpose of closing is not to only close a question after the asker has (insufficiently) cleared it up. It instead makes a difference to close a question before it is answered ineffectively.
Also, the option to only do part of the job is generally a feature here: it is OK if a user is willing to vote to close an unclear question but doesn't feel confident enough to (or can't, for any reason) leave a comment. And it is OK if a user leaves a comment to explain what is unclear in that question but doesn't also take the time to vote to close it.