The notion of a “common ground” intermediary is replete through resilient architectures. Network byte order, for instance, helps to keep the endians straight between computers without everyone maintaining a big table of architectures’ endians.
XYZ/D50 forms that common ground in ICC profile-based color management. Cameras don’t have to know about display characteristics including white point, and vice versa. And, more fundamentally, you need both primaries and a white point to transform color image data from one color space to another.