Difference between COW filesystems and non-COW when using file based dedupe

[KaibutsuX]

Can you offer a short summary of the implementation differences when using a file-based dedupe tool like jdupe/duperemove on a COW filesystem vs something like ext4?

When I first came to to btrfs I was expecting inband dedupe. So when I realized it was out of band, I couldn't understand what benefits a COW fs offered if you still have a use a userspace utility to link/delete dupes after the fact. After all I was using fdupes on standard ext2/3/4 filesystems with essentially the same results.

So what advantages (at least specifically for dedupe/COW) does btrfs provide over legacy filesystems?