An example I can think of is when you're listening to a podcast or music that has been recorded on low volume. Without a volume mixer you're forced to increase the general volume, resulting in overly 'loud' sound notifications from other apps.
Adding per app volume control is IMHO a really bad UX. It means there's a master volume and then per app volume is like a coefficient the app volume is multiplied by.
My car has its own volume control separate from the phone bluetooth volume and I hate the experience of having two volume knobs interacting with each other when all I want to do is to increase or decrese volume
I really prefer it for having different things set at different volumes. Like presets.