one word:
advertising.
I think there was expectations that it might add to battery shift life, but when you consider that a battery is not a capacitor and does not -instantly- take back into it what it took out to make the truck move, you recognize that there is no way that you gain back all the energy.
What effect it does have is to take some of the kinetic energy from the truck's motion and instead of wasting it to external heat via friction in the brakes, it 'sinks' the energy back into the battery, but that energy does not completely turn back into the chemical energy that the battery can use, a good deal of it still winds up as heat, (and some is wasted as loss in the conversion of energy from kinetic to electric to chemical) but the battery is such a larger heat sink that it is less noticeable than if that same heat was all in the brake drum.