Truck drives into aircraft wing - photo

[MiTasol]

Your truck driver was full of it.
They are to minimize the drag caused by eddy's or to cover mechanisms such as the flap tracks, or both. That particular one is for the eddies caused when the aileron, that is the movable section outboard of the landing flap, moves up and down. Ailerons move down to lift a wing up and vice versa. With very few exceptions aircraft change direction by lifting the wing on the side you are turning away from. The fin at the back provides directional stability and the rudder behind it is almost purely to compensate for cross (side) winds during takeoff and landing