"requires" is actually for when an add-on requires permission to execute certain actions on behalf of the Heroku user as part of operating the add-on. One example of that would be an auto-scaling add-on that needs to be able to change dynos. With the addition of this new API feature it will be possible to do such things without requiring the user to provide their account creds and/or API key.
This feature is still in private testing. We'll have more info on it as it's finalized.
-M