Hard drive setup?

Why is it that so many people nowadays choose to buy two identical hard drives and put them in RAID? I mean I understand that it's supposed to give faster performance, but in the end, doesn't two drives being used like that still count as only one? I don't know much about computers, but in the past I've always heard that it's always better if you use one main drive to store Windows and applications on, and extra drives for resource intensive tasks such as video editing and the like. I haven't been following computers lately but it seems like a huge waste to use two massive hard drives in RAID because say if you do video editing on those drives, aren't they still the ones that store Windows, and so wouldn't everything be slower? It just seems more efficient and less demanding on the system to divide data between drives based on usage. Am I missing something here?Hard drive setup?
The thing you are forgetting is that RAID is used primarily to compensate for seek times and rotational delay in drives, not bandwidth and processor usage slowdowns.Todays processors, buses and chipsets have more then enough power to outpace the drive , therefore it is more efficient to have two drive heads seeking; using a disk array controller, more processing power, and bandwidth than in a standard configuration. Also don't forget some RAID users use it purely for backup purposes, above I am speaking of striping.