Being “off” means that what the monitor emits is not exactly what it should have been. Too bright, too dark, too red, too whatever. There is no one “off”, every display is differently incorrect, so it would be impossible to use as a reference. The correct reference is a sufficiently accurate representation what every display aims to be, even if they don’t quite get there. That’s what calibration does: get closer to the target all displays are trying to achieve.