Except for RAID 0, all RAID levels provide protection from a single drive failure. Even when two drives fail simultaneously, a RAID 6 system still functions. You still need to back up the data kept on a RAID system for total security.
If a power spike causes all drives to fail at once, that backup will be useful.
It serves as a safety measure in case the storage system is taken.
Backups can be stored at a different place off-site. If a natural calamity or fire decimates your workplace, this could be useful.
User mistake is the main justification for backing up many generations of data. A solid system of backups guarantees you can still access those files even if someone mistakenly deletes some crucial data and this stays undetected for several hours, days, or weeks.