Using the power button to turn it on and off.
As above, that aside it’s Play/Pause for me. If you’re flicking between tracks on a playlist it’s good for previous/next. If you’re listening to an album, or a playlist in full, it’s good for play/pause. And possibly volume.
I think if you’re messing about with settings you use the phone. But if you have the Ultra set up exactly as you want it, and you’ve set your music going, the traditional remote functionality with buttons is better than virtual buttons on a touch screen.
To be honest, whilst this takes a while, I’m just starting to get used to what I’m using the remote for, what I’m using my phone for, and what I’m using the screen for.
If you sit down at the start and look at the three controllers, it looks a pain. But if you just use them, over time you organically gravitate to the easiest/most appropriate control for each function that’s right for you, I suppose.