Quoted from T7:

The main purpose of the mini-playfield is 2X Playfield scoring - just like the bell on AC/DC.
At any time you can double anything/everything by starting 2X Playfield.
So obviously one of the best times to do this is during a multiball - which they have integrated extremely well because Elevator multiball alternates jackpots between the elevator shot and all shots. So every other jackpot in Elevator MB feeds the upper playfield to assist starting 2X Playfield. A key part of what makes a mini-playfield fun is playing it while also keeping other balls in play during MB. You don't spend a lot of time up there, but the time you do is exciting as you are going for a big reward (2X everything).
I place it up there with my top mini-playfields implementations: Twilight Zone, The Shadow and Aerosmith