It used to be a required part of a package (old, pre-3.3 “regular package”, not newer 3.3+ “namespace package”).
Here’s the documentation.
Python defines two types of packages, regular packages and namespace packages. Regular packages are traditional packages as they existed in Python 3.2 and earlier. A regular package is typically implemented as a directory containing an
__init__.pyfile. When a regular package is imported, this
__init__.pyfile is implicitly executed, and the objects it defines are bound to names in the package’s namespace. The
__init__.pyfile can contain the same Python code that any other module can contain, and Python will add some additional attributes to the module when it is imported.
But just click the link, it contains an example, more information, and an explanation of namespace packages, the kind of packages without