Packs off T/O is a very useful tool at hot/high or short runways as it allows integrated T/O performance to be used giving up to 5T approx increase in MTOW depending on MACTOW. As an alternative APU to centre pack can be used to enhance pax comfort with a MTOW decrease of 100 kgs for the drag of the APU door being open.
Integrated take off performance has some limitations though such as dry runway etc. but is extremely useful on the 400F at places like NBO allowing a greater payload.