Did you ever run into some issue where a job would behave slightly different in you CI environment than on your local machine? Did you ever wish you could run just a few commands in a shell on your build machine?
These are, of course rhetorical questions. And if you're using Github Actions to run your CI jobs, you'll have noticed that this use case is not supported at all. There are some workarounds (e.g. https://github.com/nektos/act), but since they're not officially supported they can be a bit unstable. Also, even they usually don't reproduce the exact environment found on github's servers.