Note that there are a variety of RAID configurations. Cyn is likely talking RAID5.
Standard RAID levels - Wikipedia, the free encyclopedia
I use RAID0 on my home system. Contrary to the Wikipedia article, I see a nearly 2x improvement in drive performance.
RAID5 is used to increase server uptime. A single drive failure will impact performance; but will not cause the server to fail. In most cases, the failed drive can be replaced and recovered hot. RAID5 does not eliminate the need for backups.