The other advantage is that ZFS replication, unlike RSYNC, doesn't need to calculate diffs because ZFS it already keeps track of what blocks have changed since the last snapshot. This makes the entire process much faster less resource intensive.
Imagine the following scenario:
You are the sysadmin at a 24x7 company. You have a few hundred user's home directories (shared over NFS or SMB) on a fileserver that needs to be upgraded/replaced for some reason. You are tasked with migrating these home directories to a new file server with a minimum outage window.
ZFS replication procedure:
1. Snapshot, send.
2. Repeat 1 until the entire process takes less than a couple of minutes.
3. Shut down the NFS and SMB processes on the old server. Shanpshot and send one last time.
4. Bring up NFS and SMB on the new server. Make appropriate IP and DNS changes.
Total outage for the users: 10 minutes or less.
RSYNC replication procedure:
2. Sync again. Wait forever for diffs to be calculated.
3. Realize it's hopeless, Shut down NFS and SMB, Sync again.
4. Bring up the new server.
Total outage for the users: Hours.