One volume slider was never going to be enough
A computer running music, a video call, a game, and a chat app all at once needs more than a single master volume control -- turning everything down to quiet a notification also mutes the call you're on. The volume mixer treats every app as its own independent audio channel, so adjustments to one never ripple into another.
Settings that stick per app
Once you set a specific app quieter or muted in the mixer, that level is generally remembered the next time you open the same program, so a chat app you always keep low doesn't need re-adjusting every session.
Frequently Asked Questions
Will muting an app in the volume mixer close it or affect its other functions?
No -- muting only silences that app's audio output. The program keeps running normally, and unmuting it later restores sound instantly.
Why don't I see an app in the volume mixer list?
Only apps actively producing (or capable of producing) audio show up. If a program isn't playing sound, or hasn't requested audio access yet, it may not appear until it does.