In Python, when is platlib != purelib?

Question

Python has the ability to install platlib and purelib in different locations. What's the difference, why does this feature exist, and when is it used in practice?

Answer 1

To quote from Installing Python Modules:

for example, if you want to maintain all Python module-related files under python in your home directory, and you want a separate directory for each platform that you use your home directory from[.]

(I.e., you're in a heterogeneous network environment where computers may run different operating systems but each mounts your homedir from a network storage server.)