Sunlight may be needed to prime T cells to function in skinThis article explains how the skin reacts to sunlight in a way that improves it's ability to deal with cancer cells. Those with an understanding of science may care to read her research paper
"DCs metabolize sunlight-induced vitamin D3 to 'program' T cell attraction to the epidermal chemokine CCL27". however others may prefer to hear her discussing her work at Science News Central it does not take more than a few seconds to register to watch the video.
The point you need to understand is that while having a sufficiently high vitamin D status is going to help protect your body from cancer to protect your skin you need to activate the protections mechanism right where it's needed. As the sun not only makes Vitamin d on your skin but also processess it to the active subtance that does the work, having this occur on your skin is better, more efficient and more effective, than taking a supplement and having to wait for the metabolic product to be processed in the body and then travel out to where it is needed.