3) Airlines also like aircraft certified to lower weights as Airnav charges and landing fees are usually predicated on the certified Max Structural Take Off Weight.
As it is the Maz Zero Fuel Weight that usually determins the load the aircraft can carry, the MTOW generally only determines the range. If you are operating a full 737, but only on short hops, then you never get close to the MTOW, so you ask Boeing for a new flight manual page limiting the weight to attract lower landing and Airnav fees.