The major benefits of RAID technology are:
RAID can combine small disks to create a large virtual disks. This makes it possible to create large storage space using small disks without investing much on new high capacity disks.
RAID can also create redundancy. Different levels of redundancy can be achieved. Simple concatenation with parity can create a volume where any one disk can be removed without losing data.
In higher level of redundancy, disk mirroring RAID level can be used. In this level, whole disk volumes are replicated in separate disks. Failure of one whole set does not result in loss of data.
Based on the necessity, different RAID levels can be configured to achieve high capacity, high reliability or a combination of both using available, inexpensive disks.