It's pretty simple actually. Everything has weight savings on an aircraft. The more it weighs total, the less weight in a useful load it can carry. Think about the weight of a gallon of paint and a gallon of primer, times however many gallons of paint and primer just to paint an aircraft. That's the main reason they strip them to bare metal when repainting them.
The main reason American Airlines had polished silver jets and just had the wings painted grey and of course the red, white, blue stripes on the fuselage. It saved weight having polished skin instead of paint.
Imagine how many gallons of paint and primer to paint an entire Boeing 747 400 series. Now imagine the weight of all that paint and primer.