RAID is not designed to protect your data
RAID is designed to minimize or remove the downtime involved when a hard drive fails
Example
User 1 has a system with an 80GB HDD, no RAID. He backs up his data daily
User 2 has a system with 2 80GB HDD, in a RAID1. He backs up his data daily
User 1 suffers a hard drive failure. User 1 must now replace the dead hard drive, reinstall his OS, reinstall his programs, reconfigure his settings and restore all his data back his backups. During this process, he's without the full use of his machine. This might take him hours at the least, days at the worst, to finish. Not to mention all the patches and updates he'll need to apply, programs he might have forgotten about, etc.
User 2 suffers a hard drive failure. Because he has a RAID1 configuration, he simply replaces the hard drive that has died - and nothing else has been interrupted. He's down for only the time it takes him to put a replacement hard drive in. If it has to wait for a replacement, he can continue to use his system as normal - since the redundant disk is still functioning.
That is where the primary advantage of RAID comes in. We can go more detailed as to what RAID levels are ideal for what scenarios or other purposes it has (performance vs redundancy), if you want.