That's one of the success marks of MD I think; it retracts a bit from total flatness and says, ok, even though this is on a screen and will appear flat, how can

That's one of the success marks of MD I think; it retracts a bit from total flatness and says, ok, even though this is on a screen and will appear flat, how can we give subtle cues that it's actually a thing (material) and you can do stuff with it? And they decided to do it with shadow, animation, and contrast. Maybe there are better ways as well, but the end goal is sound.