In case you aren't happy with a recent update? Want to test stuff on older firmwares? If something goes wrong with an update you can downgrade to troubleshoot it etc etc
I make one anytime I make changes to the NAND and name them like {(sysNAND A9LH V. 11.0.0-33 (date) (region)}. That way if anything should happen I know exactly what backup will do what. Even if you don't
think you need it, one day you'll be glad you have them.