It adds charge to the 12 volt battery. If the 12V system is fully charged, the solar roof spins up some tiny fans that help air out the cabin, on the assumption that the sun is getting the interior hot.
Topping off the 12V system means that the 12V system does not need to be boosted from the high voltage drive battery as often. So, when you're out in the sun, the solar roof helps reduce the drain on electric-miles-available (it never makes it go
up, it just slows down the rate at which it goes
down, by some tiny imperceptible amount).
Since the solar roof is quite heavy, it adds to the weight of the car, which reduces electric mileage. Overall it's probably a wash.
But it looks cool.