MAN_Dispatcher
Let me have a go.
Put very simply, the maximum zero fuel weight is a structural limitation as defined by the a/c manufacturer. It is defined in the Airplane Flight Manual (for a G registered a/c the AFM is a CAA approved document) which sets out the the procedures for the safe operation of the aircraft. An airline must have a system for ensuring that limiting weights are not exceeded for each flight- in the case of MZFW it is usually by the use of a loadsheet, but can also be by the use of approved on board W&B systems. So, we need to calculate the actual ZFW for each and every flight to ensure we do not exceed the Maximum ZFW.