What is RAID, why do you need it, and what are all those mode numbers that are constantly bandied about? RAID stands for "redundant array of independent disks," and you may or may not need it depending on your data-storage requirements.
The biggest gain from using RAID is protection against drive failure--which, according to Google and other experts, happens a lot more often than hard-drive manufacturers like to admit. (Note that the word array is included in the acronym, so saying "RAID array," as a lot of people do, is redundant. Clearly, storage folks have a strange sense of humor.)