Why are the Xfinity car bodies scaly/bumpy?

I keep seeing these bumps/scales on Xfinity cars. What does this do? Or is it just an aesthetic thing? Mabe it's on all three series, but I've only noticed it on the Xfinity cars.

I keep seeing these bumps/scales on Xfinity cars. What does this do? Or is it just an aesthetic thing? Mabe it's on all three series, but I've only noticed it on the Xfinity cars.