If the control file is lost, the recovery catalog can step in to provide the necessary data that is required to restore and recover a database.
Additionally, the recovery catalog also stores records of archival backups and RMAN stored scripts that are used in conjuction with the target database.
The data in the recovery catalog is stored in ordinary tables and view. (In fact, it is stored in its own schema)
Using the recovery catalog is optional, however Oracle recommends it.
RMAN: Connecting to a target database with / without using a recovery catalog