Ph.D requires ORIGINAL research that makes a significant contribution to the field in which the Ph.D is issued. My experience is that a Ph.D prepares one for research and education. The training for a Ph.D involves years of learning how to conduct research and how to write about that research and the actual engagement in ever more complex and original reseach.
MD, or any other "Doctor" such as a DDS, requires one to demonstrate PRACTICAL knowledge, skill, and experience. The Doctor thing prepares one to be a practitioner in the field in which the Doctorate is issued. The training involves years of hands on tutelage under the guidance of experienced practitioners and advancement through ever more responsible hands on delivery of skilled service.