I am new to L4 and started importing my photos and making sure that I have a back-up. But as I import and save I would like to delete some of the back-ups. Some of the back-up files have a sub-folder named: Lightroom 4 Catalog Previews.lrdata, but not all of the backups have this sub-folder. Can someone explain to me the importance of this folder and why other backups don't have it? I'm nervouse about deleting one of the folders that have this sub-folder.