[Bluecortina]

Didn't the tone also serve as an audible wake-up call (not it's only purpose, I'm presuming) for anyone who might've fallen asleep in front of the box, or for anyone who thought their black screen meant their set was already off?

IBA transmitters had ‘silence’ detectors - if they detected no audio for a period (2 minutes from memory) they would automatically transmit a locally generated apology caption. So to prevent this, as the transmission staff were putting on their hats and coats before leaving the building they would output black/bars/test card and tone to the transmitter as they rushed out to their cars.