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The airfoil is shaped like a teardrop to create lift. This by makes the air on top of the wing go faster than the air below. The problem is, if the wing just ended without being tapered on the trailing edge, these two different pressures would mix, and create horrible turbulent wake, which would hold the plane back. 

This is a form of drag we want to avoid. The back of the airfoil is smooth and flat, so that the air above and air below meet without a problem.