Guido, I send the source's signal (fixed level) through the tape loop's output to a balanced line transceiver, which then sends the signal to a second system. Vice versa, I use the tape loop's input to take the signal from the second system and use it as a source in the main system. Obviously, I can use any input for the incoming signal, but a tape loop offers an additional input in situations where a preamp may have a small number of available inputs.
I have five sources (turntable, universal disc player, CD changer, tuner, balanced line transceiver).