This snap is used to test and evaluate individual pull requests for Snapcraft. It's essentially a light wrapper around git and Python virtual environments, simplifying a repetitive task of fetching pull requests and setting up venvs to use them.
So you have a pull request you'd like to test. When you visit the pull request, you'll notice that each one has an ID (for example, #2120). Use that ID to fetch the pull request and setup a virtual environment:
$ snapcraft-pr.init <id>
Now you're ready to run that pull request's Snapcraft:
$ snapcraft-pr <id> <typical snapcraft args>