What is the purpose of closing a question that has been answered?

I see a few questions have been closed as “The question is answered, right answer was accepted”.

How is this supposed to be used? Surely the idea is not to close all questions that has been answered manually?

Edit: Clarification
I mean closing, not accepting, using the “close” button below a question, between the edit and report buttons. Other reasons for closing a question are all for bad questions (duplicate, off-topic, etc.)

In what cases should questions be closed, in addition to answers being accepted?

There’s a difference between ‘accepting an answer’ and ‘closing’ it right? Closing it means no more answers accepted. The only reason I see to that is you thing you’ve got the right, best answer there will ever be. Which may happen. I’ve seen better answers come my way after accepting one. So I never use close.