Learning About RAID 6

I recently read about RAID 6 and don't understand why a new RAID level has been created. What is RAID 6 and what is its purpose?

RAID 6 is a logical extension of the RAID concept that adds stability. A host of RAID controllers support Serial ATA (SATA) disks, which are inexpensive and have large capacities. However, SATA disks generally have higher failure rates than do SCSI disks. If you have more than one disk failure in a RAID 5 array, it would take a long time to reestablish the working RAID volume just because of its size. RAID 6 adds a second parity disk to alleviate this problem. A functional four-disk RAID 5 volume requires five disks, whereas the same size RAID 6 set requires six disks. The additional disk adds more redundancy and makes the volume more stable.

—Bob Chronister