I'm not sure whether you're asking why to use a frontend vs YouTube rawdog or conflating Odysee/Peertube with a YouTube frontend. I thought they were frontends as well for a long time.
If you're asking why to use a frontend proxy for YouTube, there could be a few reasons. The obvious being privacy concerns, but other people prefer the less cluttered interface, no ads, no YouTube premium or sign in with google popups, no manipulative algorithm.
That immediately came to mind. But is there a way to have it reroute to a different platform if the video is on an alternative platform? I follow a few youtubers in grayjay that I only recently discovered mirror on peer tube. Grayjay didn't give any hints.