Properties like source/sink volume. It's bad UX if the adjustment is performed out-of-sync with the visual (volume slider) and manual (key press) feedback.
Because I change parts of the config via the UI many times a day. In my case, because I have multiple audio cards and need to change several attributes.
That's minuscule compared to how easy it is to parse config files vs to construct and speak to API. The volume slider UX is a better argument. That's the one thing I can think of where faster changes are indeed useful.
It’s not volume sliding, but close to it. Having a button to move a running audio stream from speakers to headphones to network playback on my phone is amazing.