Might I direct you to: https://en.wikipedia.org/wiki/Sony_Corp._of_America_v._Unive....
I suspect that GP didn't pick that example randomly, because youtube-dl is a pretty close analog of a BetaMax.
"You are not allowed to: access, reproduce, download, distribute, transmit, broadcast, display, sell, license, alter, modify or otherwise use any part of the Service or any Content except: (a) as specifically permitted by the Service; (b) with prior written permission from YouTube and, if applicable, the respective rights holders; or (c) as permitted by applicable law;"
This one alone gives you a whole bunch of reasons to download a video, at least in sane jurisdictions.
2) Large part of world's population can't afford subscription services
3) There's a lot of music that is not on music streaming services but is on YouTube
4) Sometimes you need the mp3 file (for ringtone, for editing, for remixing etc)
5) There are still some people who rather want everything on their HDD
A lot of Caribbean music (Soca, Dancehall, Parang etc.) is like this.
You just described youtube in general. As the old saying goes, "content is king". Look at VHS (if you're old enough to remember). As bad of a quality as it was, everyone used it. Pros would complain about how bad it looked, but the mass populace was just fine with it. Youtube is the same way. In the beginning, the quality was soooo bad, but users didn't care because it gave them endless content.
Pfft, people bought 5-in-1 DVDs. People bought 10-in-1 DVDs. Average Joe doesn't care about "quality", he wants the content.
But more than the quality for the top 100 pop songs almost all the official music videos they now put in like interludes, breaks, talking over and stuff to cut it up so it's not just the song.
Makes listening harder. for those who come to watch the video too.
In my case this often happens "in addition" to buying+downloading the song's mp3 (usually on Amazon).
ex: https://github.com/Rudloff/alltube