While you are correct, if the knob was only used for a single task, most car stereos I've used in the digital era give multiple roles to any knob(s) they may have. In the common configuration with one knob on each side the default modes are usually volume nearest the driver and either manual tuning or rotation between presets nearest the passenger, but if you go in to the menus one will usually scroll the menu while the other one changes settings. You don't want to have your volume or station change just because you needed to tweak a setting.