metadata_db_prefs_backup.json is the restore helper file that describes your library STRUCTURE (custom cols, col settings) and is used to build an empty DB, ready to hold
RESTORE metadata.
Even if metadata.db is trashed. This sets up a new Calibre metadata.db so when the Calibre check EACH book folders OPF, it has a home for ALL the metadata
RESTORE is NOT 'add'. You can't RESTORE after an 'add' and expect full recovery.
If in doubt, always ask (tell us what you PLAN) before doing. It may mean the difference between a near painless (other than the clock grind time) recovery, and a OMG
attempt