The existence of a backup file does not prove that a system can be recovered. A copy may be incomplete, corrupted, incompatible with the current application version, or depend on keys and configuration that were never included in the backup.
A restore test should answer practical questions: where a clean environment is created, which data and secrets are required, how long the process takes, how integrity is verified, and who decides that the service is ready to return to operation.
That is why I treat backup as one component of a recovery process. The useful property is not that data was copied, but that the system can return to a working state within a known time using a repeatable procedure.