Some tools are designed to have access to multiple python interpreters, and then create their own virtualenvs. Tox is a common example (although there are other ways to use tox in Actions). Less common, but my use case, is maturin https://github.com/PyO3/maturin , which is used to build rust crates with python bindings. It's a lot quicker to do them all at once rather than in different jobs, as most of the compiled code is shared.

It would be helpful if setup-python allowed users to pass an array of python versions and installed all of them (which one takes python is up for debate, but python3.7, python3.8 etc should be fine).