There are different benefits intrinsic to each protein source. Each one has a unique ratio of individual amino acids. Therefore, one protein source may provide you with more of a certain amino acid than another. By rotating, you ensure that you are “touching all your [amino acid] bases” aside from getting all the vitamins, minerals or phytochemicals that may naturally occur in that protein. Your body is constantly breaking down protein and rebuilding it or repairing damaged cells (that may or may not know of). During cellular reconstruction, your body may require more of a specific amino acid or vitamin/mineral than another, depending on the type of cell. If you were consuming protein from a sole source, your body may or may not be getting exactly what it needs.