I believe the theory is that the rough surface maintains a thinner boundary layer, meaning the moving airflow effectively sees a smaller surface to travel over, decreasing effective frontal area and decreasing any blockage effect, as well as allowing airflow to reattach quicker and earlier, with obvious benefits.
However, to me it just looks like less filler/paint/laquer on the removable sections of bodywork (which is likely to be replaced each race) compared to the higher quality sealing, preparation and painting of the monocoque that might last all season (or at least half of it!).
Various people have tried dimples - either like shark skin or like a golf ball, and found that on non-spinning items (i.e. not small balls) or in air (sharks tend to prefer high viscosity mediums like water) then dimples do not provide a benefit. I'm sure it won't stop people trying, and maybe one day (has this day already come?) dimpling will be better for aerodynamics on cars.