Treewind has pointed to one medical reason for systems of classification.

There is a tendency on the part of many people to equate terms of the system with intelligence, which was not the reason the subdivisions were made.
German scientists of the 1930s carried the theory too far, and their system was taken up by the Nazis as justification for 'Aryan' dominance; unfortunately in the minds of many their excesses clouded valid attempts at classifications needed for genetic and medical investigations.

One medical study I remember from years ago was a nutritional investigation of difficulties in digestion of cows milk by Venezuelans; the people with European blood lines had few problems, but some indigenous peoples had serious problems with milk.