If it's in their marketing, you can assume it's based on ideal weather conditions, and low-latitudes. I've done some solar-energy calculations for vehicles before as part of student engineering design teams, and I recall the results were pretty dismal. I seriously doubt a car solar roof would produce significant benefit in a place like Canada or Northern Europe as a practical range extender or a cost saver.

But maybe I could see the solar panel as a useful tool for camping excursions? The solar-panel use case appears (to me at least) to be when I want to drive somewhere far away, off grid, and where the car is going to be idle for a few days. Camping is the only use-case that I can think of that fits that bill. Although I'm sure other people might have some ideas.