If backup compression is not turned on the query will return a 0.
Backup compression will reduce the time taken to backup and restore a database.
In earlier versions DBA's may have turned this off if they had TDE enabled. Later versions can now compress databases with TDE.
You can read more about backup compression via this Microsoft article: https://docs.microsoft.com/en-us/sql/relational-databases/backup-restore/backup-compression-sql-server