Another similar thing is voip phones. These don't bother transmitting if there is no-one speaking, but the other party will get worried that the call is disconn

Another similar thing is voip phones. These don't bother transmitting if there is no-one speaking, but the other party will get worried that the call is disconnected if they don't hear any noise. So the other phone generates 'comfort noise' to indicate that all is still well. In fact, usually the comfort noise is generated to roughly match the spectral envelope of the real noise, so that when the silent party starts talking again, there isn't a strange transition in the sound of the underlying noise.