So, while most were campaigning for the shade to keep the sun down, I think another reason might be to keep the car warmer.
My wife sits in the back with our baby, and whenever I put the heat on, for some reason a draft begins to come from the roof. It's worse for her as her head is basically right under it, but if I put my hand up to the roof, I can feel the moving cold air there too. I wonder if it's because of some weird mini-weather condition up there? Like the warm air is hitting the cold air near seeping through the glass and then you have mini wind conditions? Clearly I have zero grasp on meteorological phenomena, but that's all I could think of as to why there would be moving cold air there when the heat is on, but not when it's off?
I'm guessing a shade up there would solve this, but perhaps there's another solution too, like some insulating clear plastic?