Yes, there is one use-case you're missing. If you've started a game(windowed), and, after you start playing, you want to listen to some music instead of the game's soundtrack. Instead of navigating a complex menu, you simply open the Sound Indicator and mute the game.
Perhaps this would be most useful with flash games in Firefox, which rarely have sound controls.
It could also be used for applications which use notification sounds, like Pidgin or Skype. You may want to hear the notification, but maybe not as loud as the developers intended, while still watching Youtube fairly loud.