Not only that, but if a hacker breaks in and all your backups are online, they can just as easily destroy them too. Or they can become corrupted due to a failure. Tapes are much more stable (unless you pass a magnet over them) and you can store them, as you said, offsite.
Snapshots are great for near-term recovery, and mirrors are even good for single-point disasters, but you always want tapes available in the event of a catastrophe.
Bruce