why does grill blocking work? aero or heat?

[bzipitidoo]

At first I thought grill blocking was purely aerodynamic. A quick look at our Festiva confirmed that the radiator occupies only the left half, and the condensor coil only the left 3/4s of the grill. It seemed to me that an engine compartment open to the front must be acting like a drag chute, and blocking part of the grill (the right 1/4 on our Festiva) would reduce that effect. Perhaps adding a baffle to shunt air from the back side of the radiator fan to under the engine would complete the effect.

But from what I've been reading, seems maybe the point of grill blocking is to make the engine run hotter, or perhaps both aero and heat. Block off as much of the grill as possible-- all of the grill if the car doesn't overheat. Well, if that's why grill blocking works, why not just use a higher temp thermostat? Are there also seasonal changes? Block in the winter, and unblock in the summer?

Part of what confuses me about the point of grill blocking is where the blocking is done. If it's for aerodynamic reasons, seems the best thing to do is make the exterior of the grill as smooth as possible. If its for temperature, then it's ok to put the block on the inside of the grill. Also, a lot of cars have a lower and an upper opening. If only one should be blocked up, which one is better to block?