I can't speak as to why other people use their alternatives, but if you use mpv with yt-dlp like the guy above, and which I do -- which isn't really a full replacement for YouTube, just for part of it -- then you can use stuff like deblocking, interpolating, deinterlacing filters, hardware decoding, etc. Lets me use my own keybindings to move around and such. Seeking happens instantly, without rebuffering time.

Also means that your bandwidth isn't a constraint on the resolution you use, since you aren't streaming the content as you watch, though also means that you need to wait for the thing to download until you watch it.

There, one is talking about the difference between streaming and watching a local video, and that mpv is a considerably more-powerful and better-performing video player than YouTube's client is.

I generally do it when I run into a long video or a series of videos that I know I'm going to want to probably watch.

EDIT: It also looks, from this test video, like YouTube's web client doesn't have functioning vsync on my system, so I get tearing, whereas mpv does not have that issue. That being said, I'm using a new video card, and it's possible that there's a way to eliminate that in-browser, and it's possible that someone else's system may not run into that -- I'm not using a compositor, which is somewhat unusual these days.