The bad thing about a conventional hard drive is that the controller is actually part of the drive (it's on the little circuit board that's mounted to the drive). If the controller on a normal hard drive fails, the drive might be toast. Sometimes it's possible to pry the circuit board off another, identical-model drive and put it on the one that's failed; sometimes it's not.
With a RAID array, the controller is a separate unit. Most RAID arrays are a box that contains the controller and also a bunch of slots for hard drives.
That means if the controller fails, you just send it off to be fixed, or you simply replace it. Each component is designed to be replaceable in a catastrophe, so if a drive fails you pull it out and put a new one in its place (and the controller automatically formats it and copies the data onto it). If the controller fails, you slide it out and replace it. If the power supply fails, you slide it out and replace it. Some RAID arrays even have spare controllers and spare power supplies.
Protecting data is the name of the game with RAID. They're designed so that failure of one single part won't destroy your data.
