Some youtubers put little gags that you only get when you pause the video (Jay Forman does this frequently). Others put "info blasts," where they put non-necessary info on screen for a second, so that people who are interested can read it, but others can safely ignore it (Tantacrul does this occasionally). Still others explicitly tell you to "pause the video" to solve a puzzle (Agadmator, does this so the viewer can determine the best move in a chess position).
There's likely other use cases that I haven't even thought of. Will these just have to go away due to the new pause ads? This seems like an obnoxious way to serve even more ad content, which will make the content on Youtube noticeably worse and less creative.