Even with the capability of creating portals, its still a tedious process of having to go through one each time he lands at a new house. Thats why he needs the sack, and to save on time he needs to fill it up to the brim. Thats where the super strength comes in. Some of those presents can get very heavy, which is why its a good thing Santa is stronger than any normal human. Maybe hes not at Supermans level, but he can carry all those toys during the night and not break a sweat. Its not just for the toys, however. What happens if Santas sleigh gets stuck while hes out on Christmas Eve? What he miscalculates the landing and the sleigh overturns. He cant call a tow truck. Theyre not equipped to deal with flying sleighs. Santa has to be able to fix the sleigh himself. Since he built the sleigh, he can break out his toolbox and fix any internal problem the sleigh might have, but thanks to his super-strength, he can move the sleigh himself if it gets stuck or overturned.