Vitamin D is extremely important for a healthy body. It regulates the levels of calcium in the bloodstream, and assists in the absorption of calcium into the bones. It has even been found to help with the immune system by promoting the function of the cells that kill infections in the body.
Strong bones are an obvious necessity. Not only do they protect the vital organs against impacts, they make moving around possible. One might counter this statement by pointing out that a beak is the only bone an octopus has, but this is an invalid argument: an octopus can only get about because he is underwater. Out of water, nothing can accomplish anything unless it has a rigid frame to rely upon.
The calcium levels in the bloodstream have a somewhat less obvious value. This has largely to do with keeping calcium levels within a safe limit. However, it also provides an immediate supply for supporting killer-T cells and their cousins who target various pathogens in the body. Vitamin D has been found to increase the production of natural antibiotics produced by the body to fight infection.
Vitamin D has also been tied to a reduction in the risks of heart disease and all around mortality. Some of the more generic methods of death may only be connected co-incidentally, but the fact of health support can make that connection less obvious but no less certain. Having poor health in one area, while not itself a deadly problem, can leave one less able to deal with life threatening dangers in other areas.