The advantage of a backup rotation strategy - not necessarily to tape - is that you are NOT overwriting the only backup each time you make a backup.
As IO has already said, this allows you to retrieve earlier versions. And not just earlier versions, but versions prior to a virus infection, or a data corruption (that may have happened days or weeks earlier).
The other point about backup rotation strategy is the ability to store the media off-site.
Whether you now use tape or some other medium is less clear-cut now than in the days when disk was VVVVVVVVV expensive, and tapes were not!
Also bandwidth used to be expensive - now off-site backups can be done over the wire! Snap to disk locally, archive the snap copy to the offsite mirror and back it up to tape remotely.
The other thing I would like to point out is that DVD R and RW particularly, have a frighteningly short data retention life. I read yesterday (IT week) that after only 3 years, some of the data on a DVD-RW will be corrupted. DVD-RW should be seen as "volatile" storage!
SD