I had thought exactly as you currently think. A way to measure your battery voltage. After my test, I now think it is something else. I know it is not even close to accurate.... where do you think the MFD comes up with that number -- 12.1, 11.9, pick a number, any number -- when you turn the key to the accessory position?...
It will be interesting for MF members to check their MFD reading, with ignition ON but, car not started & post their V reading.
Perhaps it is some type of artifact not meant to be used hence, Not listed in the owner's manual?
Have you noticed when you use a multimeter to test voltage for anything....b4 you connect the leads to whatever you're testing, your reading is not "0" but a small, ever changing voltage? I recall reading somewhere that it was measuring atmospheric voltage or 'floating input'. Also read it was capacitive coupling but, that may be when you connect 1 lead or probe but b4 you connect the 2nd one.
If this was the source of the MFD reading b4 you start the car, I'd expect the reading to be a fraction of 1 volt & Not
12V. If the Car's computer was capable of reading battery voltage...why would Porsche hide that fact?