The main reason is that it protects the aircraft from the effects of solar radiation. Aircraft struggle to stay cool while loading and unloading passengers at airports in hot countries, and brilliant white paint helps bounce back some of the sunlight. It also helps protect aircraft parts made out of composite materials from damage through ultraviolet radiation, which is substantially higher at altitude.
The military aeroplanes the F-35 Lightning II Joint Strike Fighter (JSF), the F-22 Raptor, the F-15 Eagle, the F-16 Falcon, the C-17 Globemaster III, the EA-6B Prowler, and the F-111 are powered by engines from which manufacturer?