It's useful because I have something important to listen to like Mumble/Discord playing at my system volume setting, and I want the music quieter than that so I

It's useful because I have something important to listen to like Mumble/Discord playing at my system volume setting, and I want the music quieter than that so I can hear people talking. iTunes 1 (16 years ago) had a volume slider to facilitate this. It seems like a perfectly reasonable thing to include even if some of their users like for music to always be the loudest thing on their computer.