The Germans were reasonably confident that Allied bombs would be unlikely to break through the roof of an armoured bunker and sink a submarine but they were worried that torpedoes could be dropped ln the entrance or bombs could be lobbed inside from low flying aircraft, so they arranged for barrage balloons to be deployed from ships moored outside the bunker entrance. It is not clear from the photograph whether the balloon could be deployed from the launch or if the launch was taking the balloon to a ship moored outside.