Kevin,
The reason is very simple; most airplanes are not designed to be water tight. When you take off or land from water the water is splashed up onto the airplane at some what high pressure. Any gap in the wing saddle area will allow water to enter. Being a high wing design the water has nowhere to go but into the fuselage, a low wing plane would allow any small amounts of water to run back out of the plane
That's why it's very important to make a gasket at the wing saddle
Especially on a high wing plane