It's pretty much throughput. The cap-ex of a single cab swamps any other cost, so the only way to reduce total number of cabs is optimizing for throughput.

Modern elevators have regen braking, so they can recover most of the energy spent on going down. Older elevators used braking resistors for that.

(BTW, going _down_ while empty is typically more energy-consuming for the elevators because they are counterweighted)

Edit: that's also why you shouldn't use elevators in a fire. If the brakes in the elevator machine room fail, the cab won't crash down. It will go _up_, possibly dragging you into the fire. Many firefighters died because of that.