Public DomainAnswer: Telecommunications
Modern telecommunication devices like cellular and landline handsets are too good for, well, their own good. Historically, there was always some line noise in the background of an analog telephone connection. Even when no one was talking and no background noises from the other end of the connection were transmitted, there was always a persistent and low-level hissing noise that signaled to the listeners that the line was active.
When telephone networks, both landline and cellular, began shifting toward digital transmission technologies, the hiss vanished only to be replaced by synthetic background noise. It turns out that we’d all grown accustomed to the faint white noise in the background and that in the absence of it, several things happen. First, people believe the call has been dropped because the line is too silent. Second, the shift from zero noise to speaking volume is jarring and unpleasant. Finally, the way noise-gate circuitry functions in the absence of that white noise means the voice transmission can sound choppier to the listener.
To combat those problems and to give people the experience they’d grown used to over a lifetime of telephone use, modern telephone systems and hardware include a small but dedicated sub-system designed expressly to introduce the missing analog white noise back to the handset. This noise is known as “comfort noise” and is even formally outlined and defined via the RTP Audio Video Profile, RFC 3389.