I use youtube-dl to archive my recordings of my own performances which have been put online by third parties (and which no one but me has any rights to!).

I use youtube-dl to view videos on other sites (such as vimeo) that don't display in my browser (due to compatibility or privacy settings), usually product support stuff.

I use youtube-dl to collect evidence about cryptocurency scammers, which often gets removed from youtube by the time the scam starts collecting. By collecting it when its online it makes it possible to hand over to lawyers so they know what to go subponea. At least once a video I saved probably personally saved me from a frivolous lawsuit (attorney fired his lying client after sending me a threat letter which I responded to with an archived video).

I sometimes use youtube-dl to buffer longer pieces which I'm watching which might suffer from infuriating connection interruptions.

I don't use youtube-dl to save copyrighted music (except incidentally), among other reasons: the audio on youtube is often pretty low quality.