[Spinnaker]

I don't think the rubber trim is there to seal the joint from water intrusion. The front edge of the headlight is not sealed and will let plenty of water in. Any water that gets in will just pass on down and drain out where the radiator air exits.
My take on what it does is to provide a cushion between the fender opening and the headlight assembly. This will prevent the paint on the flange from getting worn and the plastic edge of the headlight from developing stress cracks on its mating edge from contact with the fender.
FYI, no response yet from the restoration weather strip company.