The "weatherstripping against the cowl," as well as that above the radiator, assures proper airflow through your radiator. This according to Mr. Haines.
The physics of this is quite simple. You want high pressure in front of the radiator, and low pressure behind it. The cowl is a high-pressure zone. If you open up the engine bay to this high pressure zone, you increase the pressure behind the radiator, thus reducing airflow across the radiator fins.
