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when the car's at speed the high pressure area in front of it forces air trough the grill and the radiator, and to a lesser extend some suction underneath the car, gets the air out of the engine bay... but when it's static or at slow speed this airflow needs to be forced by the fan... and a fan basically just moves air... it doesn't create a real pressure difference...
the fan in the car is made to move air trough the original opening and trough the radiator, but if this openig is reduced by lets say 50% your fan would have to be 50% stronger in simple terms... since the grill is calculated for a worst case scenario ,under normal conditions it's to big, so partly blocking it will still deliver enough air to the rad to keep things withing normal parameters.
this is also the reason why some sportsplanes tend to overheat while when they're taxing.
their rads are relatively small and placed in housings designed to deliver the right airflow at the average speed the plane will travel... on the ground there's not enough airflow trough the radcore and things can get to hot.
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