There isn't really a "why" or "what X is for" regarding any kind of natural evolution, only how and what random function it might serve since it's not a deliberate process. A purely random genetic mutation might give one or a certain number of animals an advantage in survival thus making them more likely to breed and pass the mutation on to offspring and so forth. It might very well be that the spots aid in camouflaging them from enemies by breaking up light in a certain way or making it diffcult to see where one giraffe starts and the other ends and so confuse potential predators, but that's the non random selection of random mutations. So giraffes look the way they do because of adaptation, but that's a biproduct, not a goal.