exept for looking, I can't figure out what does it do actually.

(some are aero dynamic, but most seem are not)

 

On real race cars, its meant to smooth the disturbance of airflow along the bottom/side of the car.

I seriously doubt that any aftermarket 'kit' manufacturers have bothered to wind-tunnel test their products - though Apexi states they took drag and aero considerations into account when CAD designing theirs.

I'm not betting my life on it, but they are my favorite anyways.

I do know Mazdaspeed tests their kits in windtunnels though (hey, they have the resources hehe), and I'll have to find the pictures in one of my RX7 Magazines of them doing so (specifically the new R-Spec).