To try to answer you anyway, they've put a fair bit of engineering effort into making YT mildly painful to use without Premium -- things like not allowing sound to play when the screen is off (and covering your screen in ads for Premium if you wake up the phone to a running YT instance). I'm sure some people pay for Premium to improve that experience.
You mean for users not paying for Premium? They will almost certainly not do that, since fewer watches means fewer ad impressions, which means less revenue for them. Plus, they don't want to push away the users that spend all of their free time watching YouTube videos.
To try to answer you anyway, they've put a fair bit of engineering effort into making YT mildly painful to use without Premium -- things like not allowing sound to play when the screen is off (and covering your screen in ads for Premium if you wake up the phone to a running YT instance). I'm sure some people pay for Premium to improve that experience.