It comes from the time where TV and electronic devices could be either completely switched off, or in sleep mode (ie can react to remote control): the red LED m

It comes from the time where TV and electronic devices could be either completely switched off, or in sleep mode (ie can react to remote control): the red LED made it clear the device was not completely switched off. Nowadays, this is obviously useless as most electronic devices cannot be switched off anymore, yet the tradition remains.