The up-turn reduce the vortices (swirling air) that develops at the wing tips (I'm not sure if you want me to get into exaclty why these vortices occur). These vortices result in a lot of drag, meaning that the airplane would have to use more energy to travel at the same speed. Reducing the vortices reduces the energy needed to fly. Interestingly, some large birds have special feathers used for the same purpose.
