This level is usually implemented as mirroring. Two identical copies of data are stored on two drives. When one drive fails, the other drive still has the data to keep the system going. Rebuilding a lost drive is very simple since you still have the second copy. This adds data redundancy to the system and provides some safety from failures.
Some implementations add an extra RAID controller to increase the fault tolerance even more.
It is ideal for applications that use critical data. Even though the performance benefits are not great, some might just be concerned with preserving their data. The relative simplicity and low cost of implementing this level has increased its popularity in mainstream RAID controllers. Most RAID controllers nowadays implement some form of RAID 1. The technique used here is somewhat similar to striping with parity but not really.