Sure. Say you're building an app that leverages MailChimp's platform for sending emails. Someone that uses MailChimp could come sign up for your app using their MailChimp account, which would be validated through this library and the OAuth2 provider (MailChimp) and you'd know they were a real user and you'd have API keys to make calls on their behalf.

So now they can use their account and your app and everything is hunky dory.

I know that's pretty general and all, but that's how you'd use it. MailChimp is trying to move away from API keys and all that 'messy' authentication and toward OAuth2.