The ancient civilizations worshiped the sun and used the sun's energy to heal their bodies. Sunlight is our free source of Vitamin D, and what is better than free? As we get closer to  modern civilization, we see individuals who do not get enough vitamin D & sunlight. Some people work in environments where sunlight is absent. Others don`t get free vitamin D & sunlight for fear of developing cancer.
A "vitamin" by definition is a substance regularly required by the body in small amounts but which the body cannot make and is, therefore, required to be supplied in the daily diet. Technically the molecular species classified as vitamin D is not really a vitamin because it can be produced by exposure of the skin to sunlight, and this is the field of vitamin D chemistry. However, for nutritional and public health reasons, vitamin D3 continues to be classified officially as a vitamin.
The chemical structures of the D vitamins  were determined in the 1930s in the laboratory of Professor A. Windaus at the University of Gottingen in Germany. Vitamin D2 which could be produced by ultraviolet irradiation of ergosterol was chemically characterized in 1932.
Vitamin D3 can be produced photochemically by the action of sunlight or ultraviolet light from the precursor sterol 7-dehydrocholesterol which is present in the epidermis or skin of most higher animals. The conjugated double bond system in this specific location of the molecule allows the absorption of light quanta at certain wavelengths in the UV range; this can readily be provided in most geographical locations by natural sunlight (or UV-B). This initiates a complex series of transformations that ultimately results in the appearance of the vitamin. Thus, it is important to appreciate that vitamin D can be endogenously produced and that as long as the animal (or human) has access on a regular basis to sunlight there is no dietary requirement for this vitamin.
We all need sunlight & vitamin D. Clinical studies demonstrate the need for vitamin D & sunlight is genetically variable. At the present time many persons world-wide suffer from D deficiency or insufficiency. Testing is the only way to know how much D you have and testing is the only way to monitor D supplementation. As actions of a pro-hormone are essential to health and life,  'guessing' or 'assuming' is not a wise way to determine optimal levels.

The problem of vitamin D deficiency increases dramatically in persons living at latitudes more distant from the equator and in persons living in all US latitudes with darker skins. In Texas, for instance, there has been an increase in the number of children with black and Hispanic parents suffering from rickets. Even in sunny southern California vitamin D deficiency or insufficiency is prevalent in part due to avoidance of midday sunlight and the use of sunscreens which block vitamin D production .

Getting enough vitamin D, from sunlight or supplements, is important to health and longevity but too much supplemental vitamin  D & sunlight can be disease producing. Too much, too little - the only way to know how much D you have and how much vitamin D & sunlight you need to maintain D sufficiency is to pass a  test.