The signal loop is used on a few Freeview terrestrial recorders/PVRs, the main aerial feed will plug into one tuner input, then loop out and back into the second tuner. Some single tuner boxes will also have a loop through for enabling the RF modulator.
It can also be used on some twin tuner satellite receivers, where only one satellite feed is available, for limited recording capabilities.
You can also loop scart connections from say a video, through a Sky digibox to the TV.