This sounds a little weird but what it does is necessary to keep your database up to speed.  The Pack is built into the Utilities > Backup-Restore > Backup option and needs to be done everyday the database is used.

The Pack All will:

• Remove any records marked for deletion (entries in Red) in the Thesaurus, Data Entry, Circulation.
• Tidy up any unnecessary blank space in your database.

Alternatively you can choose to Pack only certain modules ie Circulation, Thesaurus and Catalogues.