Same physics as a golf ball, they explained it quite well on the show.

The Veyron has a very aerodynamic shape. Probably doesn't need and wouldn't benefit from dimpling. A less-aerodynamic shape, something that doesn't come to a teardrop point in the rear... like the typical sedan or hatchback (or a golf ball), has a turbulent area behind it. Disrupting the "boundary layer" or air on the surface with dimples (or something else) reduces the turbulence behind the car and produces less drag.

That's about the best explanation I can give.