A good server design has no, or very few, single points of failure. One of the most common server component that fails, are disks. So data redundancy becomes essential to recoverability. Redundant Array of Independent/Inexpensive Disks (RAID) is a disk system that provides better fault-tolerance by making use of redundancy of disk(s).
RAID 50 (mirrored arrays of RAID 5) is also a common setup as it provides increased security over RAID 10 (2 drives from both arrays must fail for all data to be lost, rather than 1 from each array in RAID 10). For databases that are critical, I would recommend RAID 50 over RAID 10, decreasing the chances that you'll need to experience downtime should a failure occur.