It's just an random number, there is nothing technical about it in todays era of digital workflow.

Maybe back in the days it had to be a certain level so background noises (from tapes, "coil whines" etc) aren't so noticeable in the mix. Same goes with having to high so you won't risk getting clipping problems, since you kinda only had one shot recoding stuff since recording etc was done on tapes, so around -6 is a safe area to work around.