Sealing any holes where air can escape is commonly done to improve cooling system efficiency. Blocking leaks ensures as much of the air as possible coming in through the grill, exits through the radiator. Of course, some of that air is also used for interior ventilation, and any leaks in the forward compartment will reduce efficiency there, too. The bonnet would have been factory sealed with a foam strip (on the underside of the bonnet) to the top edge of the plenum bulkhead, and sealed to the body around its lower edges, again to improve efficiency of this system.
Having said that, there are more elegant sealing solutions than the current tar job.
