It's not just hiding, It's also time management. With the FIA mockups, the markering team can go ahead and plan, execute etc all the launch event things without having to wait for the actual car (which are still in production). There was that McLaren docu a few years back, they were still assembling the car on the podium for the launch presentation. Mercedes the last few years took their "look at our new car" pics on the way to the first testing days, including some of the testing equipment (like infra red camera's) already on the car.
Not having to worry about PR-launch, just takes a bit of stress away from the factory. With a bit of luck we'll see non-painted cars again at the first (non-broadcast) test, all for those extra few days.