> The function should return different values for success and error

It does, and a `void` return type enforces it. Here is the trick:

> An important convention throughout the Python interpreter is the following: when a function fails, it should set an exception condition and return an error value (usually -1 or a NULL pointer).
>
> - https://docs.python.org/3/extending/extending.html#intermezzo-errors-and-exceptions

Previously, a function could return NULL but forget to call `PyErr_*()`.  With `void`, however, the function has no other choise but to properly set the error indicator so the Clinic's part will see it and return NULL finishing the convention.