Chris Pan, writing on the Facebook developer site:
The key feature of the Facebook SDK for tvOS apps is an easy way to log into apps without having to type emails and passwords with the Siri Remote:
A fast and easy way for people to log into your app and for you to provide rich, personalized experiences. To log into an app with their Facebook account, people can simply enter a confirmation code displayed on the TV into their smartphone or computer, rather than entering their username and password with the remote.
For developers who don't mind adding Facebook code to their apps, this seems like a decent stopgap solution until Apple builds something similar for iPhone users (as I assume it would be nice to log into apps with iCloud Keychain and Touch ID).