Why does vitamin D need to be exogenous?
Vitamin D is vitally important for the uptake of calcium, iron, magnesium, phosphate, and zinc, as well as for bone maintenance:


  Vitamin D promotes calcium absorption in the gut and maintains adequate serum calcium and phosphate concentrations to enable normal mineralization of bone and to prevent hypocalcemic tetany. It is also needed for bone growth and bone remodeling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults. Together with calcium, vitamin D also helps protect older adults from osteoporosis.


It also acts as a hormone, binding to the Vitamin D receptor in the nucleus of target cells which then acts as a transcription factor for genes related to multiple processes:


  Vitamin D has other roles in the body, including modulation of cell growth, neuromuscular and immune function, and reduction of inflammation [1,3,4]. Many genes encoding proteins that regulate cell proliferation, differentiation, and apoptosis are modulated in part by vitamin D. Many cells have vitamin D receptors, and some convert 25(OH)D [an inactive form] to 1,25(OH)2D [an active form].


So, the truth is far from your statement that "Vitamin D doesn't really do anything chemistry-bound, but rather just works as a hormone matching some arbitrary protein receptors or something." In fact, a cursory reading of the first couple of Google results for "Vitamin D" shows that it is broadly involved across the body in many different processes ranging from inflammation and cancer to bone maintenance and mineral absorption.

Sources:


Vitamin D - Fact Sheet for Health Professionals. NIH Office of Dietary Supplements. November 2014.
Vitamin D Wikipedia
Effectiveness and safety of vitamin D in relation to bone health. A Cranney, T Horsley, S O'Donnell, et al. Evid Rep Technol Assess (Full Rep). 2007 Aug;(158):1-235.
Vitamin D WebMD