To flip videos that have been flipped, or for watching a video in the mirror. Maybe it's 1 in 100 videos, or even less, but it's so nice to be able to just toggle a button that flips the video properly.
Not OP, but there's a lot of unscrupulous videos out there that flip video horizontally to evade content matching algorithms, and plenty of videos taken on phones or web cams that get flipped as well