Catalog Backup files are copies of the master catalog file. Catalog files do not contain image files only a pointer (path) to the original image file on some disk The purpose of the catalog backup file is to recover your catalog when the master catalog file gets corrupted or when the disk that it stored on fails.( This is why the backup copy should be stored on a separate volume.) Lightroom does not "trash" the catalog backup file. There is no mechanism in LR to do housekeeping tasks like delete a backup copy of the master catalog file. This is a manual process and you need to do your own housekeeping using finder.
Some people out of ignorance open a backup catalog and start importing new images. At that point the backup catalog file becomes the new master. If you have been using LR prior to LR6, then this was a possibility. You need to make sure that there are no original image files hidden inside the dated backup folders containing the backup catalog file.
You can toss any older backup catalogs that you won't need to recover from a corrupt catalog file or some stupid user mistake. I have gone back as far as 6 months to find correct information to correct some stupid user mistake.