It was apparently developed for drawing lines on pen plotters. Pen plotters use either stepper motors or servo motors with encoders. In the simplest case, step

It was apparently developed for drawing lines on pen plotters. Pen plotters use either stepper motors or servo motors with encoders. In the simplest case, stepper motors, one of the steppers is the major axis and is instructed to step for each new "pixel". The other motor is the minor axis and only steps when the fractional add overflows into the integer portion of the coordinate. So, pen plotters effectively have "pixels" as defined by the stepper or encoder positions of the motors.