From: RAID {Full}

The primary functions of a disk array is to increase data 
availability, to increase total storage capacity, and to privide performance 
flexibility by selectively spreading data over multiple spindles.

Data Protection - As the number of disks on a system increases, the 
likelyhood of one failing increases.  Thus, a disk array should be immune 
from a single disk drive crash.  Disk mirroring (keeping an exact copy of a 
one disk on another) is the simplest, but requires twice the disk capacity 
(and associated cost).  Encoding schemes can be used to reduce the redundancy 
required to lower ratios.

Storage Capacity is increased by placing many smaller form factor 
(5.25 and 3.5-inch) drives onto an intelligent controller which makes all the 
drives appear as one drive to the computer system.

Performance can be increased by spreading data over spindles and 
performing operations in parallel which allows multiple drives to be working 
on a single transfer request.