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First, it keeps water away from the electronics on the firewall, as well as the ignition/distributor.
Secondly, it probably wasn't needed on the sides. For the most part, any water entering along the sides of the hood, will run down the drip gutters, that are formed along each fender side.
i fitted a heavy black strip gasket to complete the seal to the front.it did not seem to make any difference in engine temp or wind noise.i am not sure if it would affect front end lift,i'll let you all know next time i'm above 140mph.
If a longer seal were needed for the hood, GM would have done so at the factory. They did not; so it isn't needed to be any longer. It was primarily for keeping engine compartment heat from entering the fresh air intake vent in the right-side cowl area.