Or if you have a Netapp with a decent support contract: A disk fails while you're asleep. The filer notifies Netapp over a dedicated POTS line. Netapp overnights a new disk to you. You find out the next morning that the disk failed, via a call from the loading dock about a package for you. You pop in the new drive, activate one of your other hot spares, and configure the new drive as a new hot spare, all in less time than it took you to walk down to the loading dock and back.
 You don't have single disk failure alarms wake you up in the middle of the night because you configured your array to run with two failed disks.