Camouflage mainly. That and when you have to do a snap decision as to what aircraft is enemy or friendly, a blur of color is harder to recognize than none at all.
When you consider the variety of wing markings of Germany and her allies, none on the tops of your own makes perfect sense.
There is an excellent example of this at the U.S.A.F. museum in Dayton Ohio. They have a board mounted up high, with various national insignia on it. British, French, German, Russian, and the early American types.
When you are close, you think, "Shoot, I could see the difference", but when you go back to the designated viewing area, (some 100 feet or so), it is almost impossible to tell the difference on any of them. A German cross is practically the same as a roundel.
Now, try to do the recognition in the adrenaline pumping arena of combat.
This is why the "Bar" was added to the American insignia.
It broke up the blur.