Python comes with a very nice feature that enables each module to self-test. Making use of this feature is a great way of making sure that the changes to our code do not break the existing code and, additionally, the __main__ self-testing section can serve as documentation for how each module works.
After a few months or years, we sometimes forget what our code is doing, so having an explanation written in the code itself is indeed of great benefit.
It is a good idea to always add a self-testing section to every Python module, when possible. It is sometimes not possible but, in most modules, it is possible to do so.