Found on another forum, originally posted by someone with bug knowledge:
"Most flies lay their eggs in water. When you wash your car, then polish and wax it, the surface (especially on dark colored cars) looks like a pool of water to the flies, which attracts them as they attempt to lay their eggs. Though dark colors will attract more flies, light colors do as well. It's the reflectivity that the flies are attracted to more than the color.
The reason that birds will always target freshly waxed cars is still a mystery to science, I'm afraid."
So although mildly annoying when there are flies around, you could argue that the more flies you have the better prepared your car is, and subsequently it is a bizarre measure of how effective your polish/waxes etc is/are!
