It's almost as if different human populations separated by insurmountable geographic barriers in wildly differing ecological circumstances for myriad millennia underwent selection pressure!
Purity is a bit of a misnomer because a "perfectly" homogeneous species with no distinctive heterogeneous groups would have to be called pure, but that's not usually how people use the word. The implication is usually that there are several distinct, homogeneous groups within which there is homogeneity among its members.
Having homogeneity at some level, even if it ends up being in the form of several homogeneous groups, allows one to make useful scientific findings and generalizations. On the other hand, if everything was very heterogeneous, down to each individual, it would be very hard to find a general statement applicable to any appreciable number of individuals.
and yet it is possible we will need to end up where individual advice is the only one working
Is there purity? No, not in the absolute sense. However, we're not some homogeneous mix either. Genetic, environmental and other such patterns do form a number different subgroups which while imperfect, remain useful.
/s