Our normal landing flap is 40, but 30 is used (runway permitting) in turbulent approaches especialy in thunderstorms. Flap 30 has more excess power available for coping with sudden downdraughts than 40 and more so at MLW.
Speed controling at 40 has a wider power-envelope so its easier to maintain an IAS than 30 on final especialy at low weights.
Check the LD required flaps 40 and 30 for the same weight. Youll find quite a marked difference.