I can operate physical buttons without taking my eyes off the road.
I know where the volume knob is and I can easily grab it and turn it without looking away. If it's slow to respond I can go again and dial it back down, without looking away. Or I can just keep my hand on the dial and spin it back a bit.
With a digital screen I can't leave my hand on it without continually pressing the input, and if I want to put the volume down I have to look and see where that button is.
If it doesn't change -> change it more
It it changes too much -> change it less
If you realise it needs to be moved slowly -> move it slowly