Edit: let me be more clear. When do you have situations where duplicate data is stored on the same disk and the best way to deal with it is through the filesystem?
If you're talking about a webapp with lots of users uploading the same photo or something, isn't that better handled before you hit the filesystem, so that you have dedupe over a number of independent disks/locations?
But the great thing about this is that if operates at the block level - so if two people take a CAD drawing, change a small bit and save it to their home directories, most of the similar data can be stored once and only the changed blocks stored separately.
Database dumps as backups.
Email servers with lots of CCing.
Although it's probably more for 'Enterprises' than web apps.
The real value (and the content of many EMC sales pitches) is with backing up and archiving data. EMC often argues that most "corporate" data seems not to change very much over time.
I don't see how version control avoid file duplication. When you work on multiple branches at the same time. You necessarily have to retrieve each branch in different directories.
E.g., if you store dated snapshots of content, it's much easier on you if you can just store the whole directory, rather than manually maintaining hard links back to previous versions.