You never have 2 applications outputting audio and you want to balance them? Sometimes I'll be playing a game and listening to a podcast at the same time, and I want the podcast to have the louder audio, so I turn the game down.
On my Mac I use for work, sometimes I'll be in a Teams meeting where I only need to be partially paying attention, and I'd like to be able to turn the Teams volume down and have something else at the forefront, but I can't do it because there's no volume mixer.