Having a (fixed) shortcut to Genres from the main Android Auto screen. I rarely know what specific album/artist I want to listen to, only that I’m in the mood for “genre X”. Currently there’s no easy way to do this in the car.
Having to remember some album in the huge collection that matches the genre you feel like listening to, then having to manually browse to it (while driving… eek).
Quicker (direct) access to a specific genre - saves time, and safer too (because voice commands are pretty unreliable to say the least).