Relatively few filesystems offer thorough data checksumming. Hardly any offer erasure coding. RAID at the filesystem layer is a bit more common, but also more inconvenient. Doing erasure coding at the file archive level rather than in the filesystem gives you the freedom to move your archives onto standard everyday filesystems and devices without silently losing the protection.
What if I use multiple computers and multiple operating systems want to be able to work on this data on them. Par2 would allow me to create the needed data on any computer and test it on any computer.
The other alternative is having a server that's up and running all the time, exposed to the internet (or complicate the setup with a VPN), so I can sync. Operations would take a long time (via the internet) or I would anyway need to transfer the data to my computer, work on it, sync it back. During this time, any protections that ZFS offers are null since anything could happen in my computer and I can't test for it locally.
ZFS is great. But it's not the answer to everything.