Sometimes they inadvertently delete entire directories or even volumes. It's one thing if the deleted file held a short memo. It's another thing if those were the figures for the shareholders' meeting tomorrow or an entire day's worth of data entry. Sure, people can print files so they always have a hard copy, but then someone has to re-key all that data. And a hard copy does nothing when a program's executable files are deleted or corrupted.
Humans make mistakes.
So does hardware. Even though hard disks often have a mean time between failure greater than 100,000 hours, the server disk could experience a head crash, rendering your company's vital information nonsensical. There are plenty of shocks great enough to crash the heads, even though disks can withstand several Gs of force. The power does surge and sag, blowing unprotected disks left and right. Network operating systems have glitches.