1. I download whole channels if there's a series I want to watch.
2. I download music, since youtube is a really quite nice music library.
3. I download long videos before I watch it, because it's just more pleasant to rewind the thing when it's already on your SSD. Also, it won't freeze spoiling a beautiful scene because of some stupid WiFi problem.
4. Even if I don't download it before watching, I still use mpv to watch it, which uses youtube-dl internally, because mpv is a much nicer video-player than what you can have in a browser.
And I'm sure I forgot some important use-cases. Browsers are evil, js-apps are stupid, don't use them, if you can.