There's a few things. First off, ZFS takes advantage of doing its own volume management on bare metal. Some aspects of data recovery/resilience work better whe

There's a few things. First off, ZFS takes advantage of doing its own volume management on bare metal. Some aspects of data recovery/resilience work better when it can interact directly with the device, particularly for resistance to bit rot (which is one of ZFS's biggest advantages). Putting something like LUKS in between isn't as bad as LVM or hardware RAID, but is not great. Second, it enables the use of different ways of using a particular block cipher like AES (namely GCM instead of say XTS) that have some advantages such as authentication of data. I'm not sure if it's an option for this particular implementation, but nothing about the way ZFS encryption works would preclude using XTS, while GCM doesn't really map well to block device encryption (there is no good way to store the extra IV and authentication code, while ZFS can put it in the metadata). There are of course disadvantages. Some information about the structure of the data on your drive is accessible that would not be if you used block encryption. Also, unlike LUKS or LUKS w/ LVM you can't easily mix filesystem types on the same drive set.