Total speculation: Maybe redundancy for low-level controllers, doors and locks in case of complete battery pack drain. If you zero out the main cells this may be necessary to control the basic systems so you can actually restart the car or get it to accept a charge.Back up to do what?
Yeah, they actually mention this in the manual!Interesting EV efficiency fact. In most EVs the climate (fans,ac,heat) is done by the main battery but the seat heating is run from the 12v. So theoretically you could get better range by using seat heating and less climate.