The idea is that at the start of the day, a stepper doesn't know where it is.
Whilst it was powered off, someone could have manually moved the shaft, so stepping can only ever be relative to an initial position.
Disk drives with stepper actuators used to have a little crank arm attached to the shaft that on power up, the stepper would rotate until the arm interrupted an interrupter LED/phototransistor pair.
So, now the stepper controller knows where track zero, or twelve o'clock or whatever, is.