Especially streaming that you have to pay for.
It's convenience and being advertising free.... I don't want to have to find and store music for it to be ad free. If I'm on a road trip, or during my commute, or whatever - and I decide I want to hear something specific or new - "Hey Google - Play " Bam - there it is. No hassle, No wait, No searching, No ads...That's what I'm paying for. And with YT premium, you're also ad free on Youtube itself, which is it's own killer feature when you're watching on a smart tv, or you're at a hotel where many now have the ability for you to log in to the services you have on their TV with no hassle.
You can tune a piano, but you can't tuna fish. You can tune a filesystem, but you can't tuna fish. -- from the tunefs(8) man page