Imagine you have a single code base, which has a few different applications within it... or at least the ability to run a few different applications. Or, maybe you're Google with your mono repo?
In any case, how do you manage this on Heroku? You don't. Heroku applications assume one repo to one application.
Enter the Multi Procfile buildpack, where every app gets a Procfile!
PROCFILE=relative/path/to/Procfile/in/your/codebase, and of course:
heroku buildpacks:add -a <app> https://github.com/heroku/heroku-buildpack-multi-procfile
git push email@example.com:<app> master
Copy the snippet above into CLI.