A farm configuration backup will back up the configuration data from a configuration database, including settings for information rights management, email, anti-virus, and so on. This is only farm-level configuration and is intended to handle failures that involve the configuration database, but not other SharePoint components.
Appropriate use cases include replicating a standardized farm configuration and moving configurations between environments. When using the Central Administration GUI, you can only back up the local configuration database (local to the farm whose Central Administration website you are on). With Windows PowerShell, you can back up any configuration database on the local farm, on a remote farm, or from an unattached configuration database (one not associated with any farm).