Used when one wants to end an annoying conversation, where it is obvious the other person is wrong, but either one is unable to convince the person, or the matter is so trivial one can't be bothered to continue. This lets the person know the conversation is over, but they have not won.
A: Lady Gaga actually has a lot of musical talent, underneath all those ridiculous costumes and stage shenanigans.
B: Agree to disbelieve. Now shut up.