But having 2 sliders lets you change the volume per app too, not just having a global system volume, which is nice if you want to hear Norwegian Metal at full blast, but don't like to be interrupted by BS system notifications at the same volume, and so on...
Most programs, for example Spotify, then provide another slider on top of that.
A notable outlier is foobar2000. When you change the volume slider in the app, it actually changes it's mixer volume.
It's nice though if things aren't the same loudness. You can also now listen to multiple apps at the same time with Audio Focus. Pretty amazing.