I have a massive movie collection, mostly from storing the full raw DVDs on my NAS for later playback. This takes up about 5TB worth of space, and no single hard drive holds that much data. RAID allows me to combine hard drives into one massive storage pool.

I also don't see the need to back this up, since the DVDs are still around. RAID gives me a little safety net in that I won't have to rerip all the DVDs if one hard drive tanks.

RAID is also a performance boost in most cases, allowing heavy I/O workloads to be met.

The reason we constantly say RAID is not a backup is that people sometimes believe it is, and save their only copy of some data to a RAID.

Oh, and most RAID systems can rebuild while still remaining online. So even if failure strikes, someone can keep working on the system while it recovers with just a little bit of a performance hit. My last job had everyones desktop set up with RAID for both performance and the ability to keep them working if one drive failed. Recovery from a complete drive failure would take more then just a hour in that environment. OS load, + bunch of apps + restoring work from the version control system would easily chew up a productive day.