There's some dead air when doing narration like tutorials so it would be nice for the viewer to listen to some relaxing music or to listen to the original audio when I'm done speaking when the audio level of that narration dips below a threshold.
Also, it would be useful for when scoring an action scene and you would want a diegesis audio to be played during the loud fights.
I hope this makes sense

I heard about this feature being used in a game called, F.E.A.R. 2.