Because sometimes you just want to run different configurations of the same app on Heroku. See buildpack-procfile-select-test for an example.
You almost definitely want to use this as part of a multi-buildpack.
This buildpack must come before your actual environment buildpack (when the process starts).
A sample node.js .buildpack file may look like:
https://github.com/jessefulton/env-procfile-buildpack.git
https://github.com/heroku/heroku-buildpack-nodejs.git
Then if you want to use a different Procfile than the standard one, simply set an environment variable, and push!
heroku config:set PROCFILE=Procfile.dev