The human body is not just a collection of organs and organ systems that manifests symptoms and signs only to indicate the presence of a disease.
Indeed, our body is a complex machine that functions in such a synchronized and coordinated way. This is so amazing that its functions cannot be replicated by any super technology. To unravel the mystery of how each cell and its organelles functions within and among themselves is beyond reality despite so many inventions and discoveries. And more importantly, how our genes are regulating these cells is yet another miracle.
And you know! these genes and cells respond in numerous ways to each food & nutrient. This means that the way these genes and cells behave to various nutrients present in our food and ultimately projecting their results on our health status forms the basis of why we should incorporate nutritional science in rendering health care. Food and water are the most basic needs for human survival. Even though we function through the outer world with the help of the solid body structure, every vital function is being carried over at molecular levels that cannot be seen or felt. The molecules need the right chemicals to drive them. The right chemicals are present in the right food. Every machine needs fuel and so everybody needs its fuel in the form of nutritious food. That food should support our survival, repair, and renewal. Tapping this science and knowledge about food and nutrition helps us lead a disease-free life. 
It feels even nicer to hear "live with a good quality of life". 
Quality of life is more meaningful than its longevity. The one life we live in this birth should be healthy, productive, efficient, self-reliant, and satisfactory. We all want to live without suffering. But we are not aware that we are harming ourselves with the wrong food choices thinking that we are living a pleasant life by enjoying the foods that have no nutritional value. 
Without knowing how our foods kill us and treating the symptoms of the ailments with medications is like scratching the dirt from the surface. But that doesn't solve the issue of why and what caused the problem in the first place. i.e., without clearing the root cause of the issue, you can never help a person how so ethically you do your profession. People should be taught how to eat right as that is the only tool that is going to keep them healthy till death.
Everyone has the right to know what to eat right!
The scope of the physicians should be to teach their patients how to make their food-friendly for their bodies.