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because air stagnates there. And that high pressure air creates lift (not desireable unless you're making a flying Z). Precisely why racecars have fender vents (the gills in the fenders above the front wheels) - to vent that high pressure air: 
Anyway, back to the Z... You've got high pressure in the wheel wells (because the air is relatively stagnant) and lower pressure outside (because the air moving over and around the car creates a lower static pressure). Naturally, air will want to flow from high pressure to low pressure (out of the wheel wells). Also, a decent amount of the heat from braking is transferred to the air in the wheel wells. And we all know what hot air wants to do - expand. In this case it wants to expand and get the hell out of the wheel wells. So, you've got two things already working to get air out of the wheel wells: (1) a pressure difference and (2) expanding hot air. Nissan figured "why not add a third" and integrated a fan effect into the wheels that also helps move air out of the wheel wells to help cool the brakes: 
 "The more "F" and the less "m" you have, the more "a" you can get." -- Brian Beckman, The Physics of Racing |
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