Makes the cars spin better. When I go to see the stunt shows st the fairs here, they always wet the surface. That way the cars will power slide and such easier.
I am in Film school and it is best to shoot any exterior roads wet. The reflection it gives is beautiful, plus it gives the cinematographer more to play with.
Graduated film school (BA in TV/Film)... along with wet roads, you'll often see overcast skies--great for getting true color on film. This applies mostly to car commercials.