Returning to the lobby means you're in the right place half the time (roughly -- I for one use the elevator to go up more than to go down). Anywhere else has much worse odds.
This is easiest to observe in builds with glass elevators.
It's too bad there aren't more games in the genre. It might be fun to experiment with more types of traffic flow and programmable elevators.
If the elevator is needed at the lobby 50% of the time, means it is also needed elsewhere 50% of the time. It means that 50% of the time the lift is travelling to the lobby and then back to a needed floor empty. If it only moves when called then it only ever travels the difference between its end point and new start point which is AT MOST the number of floors in the building.
If every floor is accessed equally (as you'd expect), then 50% of the time the elevator needs to travel half the height of the building (on average) TWICE when it goes into idle state. The average of this method is equal to the maximum of the other...or something.
....but then I haven't had my morning coffee yet.