It happens: you start using a module in your project and it works and you don't realise that it's only being included in your virtualenv because it's a dependency of a package you're using. pip-missing-reqs finds those modules so you can include them in the requirements.txt for the project.

Alternatively, you have a long-running project that has some packages in requirements.txt that are no longer actively used in the codebase. The pip-extra-reqs tool will find those modules so you can remove them.