Why exactly GM installed these on hundreds of thousands of trucks, I don't know the specific answer.
I can say this though: when you consider that automakers, including GM, spend countless engineering and research hours into whether or not to do a particular feature/modification...the reason might be to save 5 cents per vehicle on the assembly line, or maybe to improve gas mileage by 1MPG, or...maybe to keep engine or transmission temperatures at a correct level. Regardless of the reason, there IS a reason, you can rest assured.
So in other words, there's a good reason GM sticks that piece of plastic on the light duty GMT400 trucks. Whether we can comprehend that reason or agree with it is irrelevant.
Personally, I like 'em. When I see a truck without one, I think, guess it got broken off - looks incomplete.
Richard