Some day, when you have half an hour to spare, press M-x ibuffer RET and then h. Then take your jaw from the floor and put it back in place again.
One use-case: if you sometimes e.g. visit a lot of files (e.g. by means of using follow-mode in a buffer full of grep results), and want to kill their buffers (e.g., because OCD – other than that there is (almost) no reason to do it!), you can use ibuffer’s filtering and marking commands to do that. Another one: query replace in multiple buffers.