Tape has a tremendous density advantage over disks. Think about how much area a tape occupies if you were to completely unroll it, and compare that to a disk platter. The flip side is access times are extremely long compared to disks, but for the usual application of backup and recovery that doesn't matter much. In this case they're pairing the tape with faster non-volatile storage so writes at least will be very quick (reads not so much, 2 minute access time!)