The power does surge and sag, blowing unprotected disks left and right. Network operating systems have glitches. Mysterious system errors can cause files to disappear. Viruses don't just make good headline news stories. They are rare, but when they appear they can be responsible for vast amounts of damage.
Natural disasters do strike.
Earthquakes, floods, and tornadoes really happen. MIS departments wouldn't dream of running minicomputers or mainframes without backup systems and procedures. A PC network is no different. The simplest method, to back up to floppy disk, is really only suitable for standalone PCs with small hard drives. If a server has a mere 40MB drive, and 20MB are used, it would take 14 floppies to back up that information, and that's if you use 1.44MB diskettes, the highest capacity available.