People should do it in some cases because sometimes Python libs have to link against some other non-Python libs that are not part of the pip package. Then if y

People should do it in some cases because sometimes Python libs have to link against some other non-Python libs that are not part of the pip package. Then if you just install the dependencies, sometimes the newer version in the PyPI requires a newer version of the dependencies, so you are stuck installing those things from source and building packages.