Why Grand Fleet destroyers could not simply escort convoys

Destroyers were vital to the Grand Fleet's ability to fight the High Seas Fleet, so assigning them all to merchant escort would weaken the decisive force. Recalling those ships could also run into fuel limits and would abandon merchants that had sailed in expectation of protection. The older belief that an unescorted convoy merely concentrated targets came from experience with surface raiders. A submarine conserving its limited torpedoes needed to attack on the surface, where several armed merchant ships could mass their fire against it. Even if the merchants lost that fight, damage that prevented the submarine and crew from returning home changed the attrition calculation.