Good reading for you -> http://en.wikipedia.org/wiki/RAID#RAID_1
RAID 1 is strictly a redundancy scheme, not a backup scheme. There is a huge difference.
For instance, if your OS or controller starts writing bad data to the primary drive and destroys it in the process, that same bad data will be written to the mirror. A backup, on the other hand, is a selective or automatic process to a separate drive (preferably on a separate controller) that backs up your data.