Imagine a catalog with 5000 photos, all of which were edited, cropped, etc. Now your catalog LRCAT file somehow gets corrupt and you cannot recover it. ALL, all, all your edits are gone - forever! So, when you exit from Lightroom before closing the computer, the operating system closes the LRCAT file which is a lot safer. Second, change the catalog preferences to ask you to make a backup "Every Time Lightroom Exits" and be diligent about making backups to a different hard drive or the cloud (You can change this the first time on the dialog when you are actually making a backup and then LR will remember it). Any time you edit photos, make the backup. If you do not make any changes, simply click the "Skip this time" button. This is not optional as far as I am concerned - it is required to be safe!