The air boundary layer is the thin layer of air right next to the surface that is stationary relative to the surface. As you move further from the surface you encounter air that is in motion. Typical boundary layers are less than 1mm thick.
I dunno, i think it makes sense. I dunno what an air boundary layer is, but it seems to me it would have less friction for the reason basjoos said of the sprinkling of sand on your car, but I suppose pixie dust in the gas tank might be an easier way to improve FE than waxing on a regular basis.
To put a picture to what basjoos said
At the point of contact, there's a "zero slip" condition. A tiny bit above that point - there's a tiny bit of air movement.
And easy way to feel this is to open your window and put your hand against the side of your door (while moving at faster speeds). You shouldn't feel too much in the way of wind.
In any case - nature knows perfectly smooth objects are not efficient at moving through fluids... Think shark skin and look at the bumps on whale fins Then look at dimpled golf balls, fuzzy tennis balls, and how frequently baseballs are changed in game.
Keeping your exterior in good condition will prevent value losses should you decide to sell your car... I guess that's beneficial...
Time is the best teacher. Unfortunately it kills all its students.