Lack of ads. Background play. Offline viewing. Prenium members are also much more valuable for creators in terms of revenue.
I'm aware of NewPipe. But I don't like it's design and I use my YouTube account on multiple devices. I depends on my playlist, like videos, history and recommendations far too much to use NewPipe.
Also, YouTube Music is a proper streaming service with proper algorithm and so on, it's not just the music videos of YouTube. :)
And that, kids, is how Linus fucked his Pop!_OS installation.