That's because when flying close to or at the speed of sound a shock wave forms at the nose of the airplane. NASA researchers discovered that a thin airfoil delays the formation of the shock wave. This reduces drag that is caused as the airplane moves through the shock wave. Their investigations gave results that are still used today to influence the design of new aircraft. Wings provide the majority of the lift for an airplane, but wings also cause drag.
Air flowing over the top of a wing also tends to flow inward toward the fuselage.
Meanwhile the air flowing underneath the wing tends to flow outward. As these two airflows meet along the trailing edge of the wing, they form a rotating column of air that extends from the wingtips. This is called a wingtip vortex. These are visible from a passenger's seat next to the wing on humid days, cold, moist mornings or flying through mist. Energy is lost in the process of making lift because of the airflow around the wingtips.