As you can set this per filesystem one can create one large metadevice out of e.g. many Raid devices and create "low security" filesystems without ditto blocks and and "high security" filesystems with instant copies spread over the different raid arrays. Even if one whole Raid Array fails, the filesystem with ditto blocks enabled will still work as normal.
This adds further levels of data protection to really important data as well as more granularity to chose how "secure" your data should be.
Some Maths about what combination of raidlevels and copy levels is best suited for your needs can be found within the posting linked above."