In Left 4 Dead 2 you can make your character speak by clicking in the thumbstick on controller, or pressing Z or X when using a keyboard.
When looking at an item the quick voice line changes from generic "look" to a more specific "ammo here", "weapons here" or similar. This can be useful on multiplayer campaigns where it can be used to communicate with other players if a microphone isn't available.
But on single player campaigns it doesn't seem to serve a purpose. Identifying an object doesn't immediately give a voice response and there isn't much else in the way of feedback.
Is there any benefit to using quick chat in single player?