> We don’t. We just want to control the level of audio out.
You can … inside your applications.
> The end result is the same by controling system out bus or sending it to a mixer and controlling it there.
No, learn about leveling.
> One of those desk volume knobs would do I guess.
But you would never control the output volume of a professional audio device with your system settings.
This consumer feature would generate serious leveling problems to all pro users …