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Another classic example would be the "tone" sounds you hear when entering digits on a modern cell phone. Completely cosmetic, but probably just makes sense for consumers. And there's just something pleasant about hearing those tones rather than a generic beep (or nothing.)

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On dialing they might be cosmetic, but the tones are still used to communicate with remote services. When the machine says "Press 1 for English," it's relying on getting the right tones (1209 Hz and 697 Hz in DTMF). Sure, they could be sent in the background without letting the consumer hear them, but they're still being generated.

http://en.wikipedia.org/wiki/Dual-tone_multi-frequency_signa...

Except few modern devices actually transmit there frequencies in-band. They get encoded and sent out-of-band, then recreated (if needed) on the other end.

Arguably these have a function; consumers got used to them from multi frequency dialing in the past, so if you're used to the tones it provides an additional source of error correction (for example to find that you pressed 5 twice instead of 52 which you intended). The frequencies are there for historical reasons, but they do have a purpose.

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