It's useful to have some sort of baseline to use during calibration, and it's very important to calibrate speaker levels in a surround system. It's less critical to have a baseline that matches other people's, at least as far as simply listening to and enjoying your system - but the purpose of having a standard reference level is so that you can set the volume to that position and theoretically have the level that the sound engineer had in mind. (I don't know how many folks actually listen at that level, though...)