I assume the flat panels have an aerodynamic effect like the underbody of race cars. They ultimately create forces sucking the surface into a direction. And since on the sides it will be never stable it will flap around all the time. You can see that the most with the vertical fin stabilizer of Formula 1 cars. https://old.reddit.com/r/F1Technical/comments/nd2ayw/alpine_flexible_rearwing/

Here is a lot of wobbling and while the vertical changes are intended, the horizontal ones surely aren't and they tried to make it as stiff as possible. Certainly nothing a production car would achieve.

Correct me if I am wrong, as I didn't study this particular area.