MKV is the format that hoards: video, a dozen audio tracks and every subtitle, all in one box. It was born in 2002, the open-source Matroska project, and its real weak spot is playing on TVs, iPhones and editors that never learned to open the box.
Video is the heaviest thing most of us ever attach: screen recordings, camera clips, whole talks. Stands in for any video format Flippy can read: MP4, MOV, MKV, WEBM and the rest. That makes it the right destination when what you need is any moving picture: recordings, rips, exports, downloads.