Imagine you have an large Oracle database server (or any DB server) which has several large databases running from it - say they are 50GB each.
If you back them (usually) as full backups every day then after say, five days you'll have 250GB of data on tape.
If you back them up with a de-duplicating VTL after five days you'll have 50GB stored on the virtual tape. (assuming there are no changes day to day)
Of course, in the real world there are changes, but the VTL will only write the changes to its storage and track these changes in its own database.
That's one example - there are many others of course where dedupe makes perfect sense, both operationally (you only store deduplicated data so if replication is required then you onlly replicate a fraction of the backups) and from a cost and admin perspective (keeping petabytes of data on tape in huge silos plus the manual costs of copying them to offsite storage.)
<pulls up comfy chair>