You forgot about chasing and facing the BB - many people who do their own work don't own the necessary tools.
And the "calibration" process you refer to just means that the unit is performing correctly in reference to itself, unlike PT's and SRM's whose ultimate accuracy can be verified with weights.
And the "calibration" process you refer to just means that the unit is performing correctly in reference to itself, unlike PT's and SRM's whose ultimate accuracy can be verified with weights.