The passenger can use the phone if you want to be able to scroll through an artist's catalog, rearrange playlists, etc. CarPlay's UI is a quick way to access your current library.
Tinfoil hat time: CarPlay was created as a 4D regulatory chess move, because it enables users to continue interacting with their phones in a legislative environment where said action is otherwise illegal.
It would be hard to argue that it’s illegal to use the phone in a way that’s nearly identical to the way you interact with your vehicle controls.