Your engine needs air and fuel to make it work.
More air and oxygen going into the engine means more efficient combustion. With Airflow, air is taken in at roof level where the air is less disturbed, contains less contaminants from the road and traffic conditions and is cooler than air from the engine bay or at ground level.
Where air is taken from under the bonnet / hood or the wheel arch, the air is hotter than the air collected by a snorkel at roof level. The hotter the air, the less oxygen there is available for efficient combustion.