Why delete a question once you know the answer?!

Why delete a question once you know the answer?

ive noticed recently that some people do this, why not leave it on there and pick a best answer?
is that not what your supposed to do?

2 days ago
to people who says tehr isnt a rules where you have to pick a best answer, yeah, but it would be a decent thing to do, plus it cost 5 points to ask and you get 3 back for choosing a best answer.

I know what you mean :/ annoying. Or when you're in the process of giving a detailed answer only to find out the question has been deleted.

I guess it's just easier for them to delete it rather than award a best answer. Blah :/