If you just read the side panel of a Wheaties box while eating breakfast in the morning you'll already know you need vitamins A, Bs, C, and E. The milk carton next to the cereal you know is a good source of calcium and vitamin D. Why should you know about anatomy and physiology?
Understanding the whole picture is, of course, why we break down a complex anything into its parts, including an organism. Cut the body apart into sagittal or transverse sections to discover its cavities and viscera and you learn quite a bit about how a little morsel of might end up in the mouth and through the gastrointestinal tract.
As you read these words, you begin to understand the direction of the point of this paragraph. Just as it is the job of a writer to understand how words can be used for impactful sentences that create powerful paragraphs, the nutritionist can use the alphabet of nutritional elements for supporting his or her patient's body at optimal levels.
The anatomy-physiology nutritionist considers bodily needs from a chemical and cellular level as well as analyzing detection and responsiveness of an organ-system. That kind of nutritionist is one that brings holistic results to a patient, one that does good and no harm, one that can be trusted, the type of nutritionist I aspire to be.