Hi Richard K.,
(not that I suspect you'll read this, but perhaps others may) --
The above two responses indicate what makes, and why, skin pigments exist. But they don't address why it might be advantageous to have LESS pigment. That's b/c humans need to convert vitamin D2 into D3 in their skin, to have it exert maximal beneficial effects in the body (and it does have many effects!). That requires direct sunlight, and it's believed that dwellers in northern regions, where sunlight is less direct, hence weaker and much less effective for the D2->D3 thing, lost pigmentation in order to have enough D3 made to stay healthy.
There are other erroneous theories of why white skin (or people) might excel. No evidence for such theories, however.
-- Cheers, -- Mr. d.