Library Maintenance:
Rebuild reads those files and creates a new metadata.db.
AS LONG AS YOU NO MESS WITH THE LIBRARY
OTOH if you do something dumb (using Calibre), like deleting all Tags.
Those OPF files start updating immediately. (see Library Maintenance:Metadata backup status: <it
shows remaining to be written> count> )
Do NOT use the button on a damaged DB, That forces the BAD DB's values to be written to OPF's