It’s the “default” way to create Replication Tasks.
The rationale goes something like: “The user wants to make regular snapshots of a dataset. ZFS replications use snapshots. Might as well have them tether a Replication Task to a Periodic Snapshot Task. Nice and simple.”
Yeah. I know. 
It’s been like this on Core (still is).
I’ll borrow a quote from another thread:
Disclaimer: I’m not saying to use the command-line or avoid the GUI for ZFS replications. I made this personal decision (back on FreeNAS 11) to bypass the GU for replications, since it put a bad taste in my mouth from the very start.
I could possibly switch to the GUI now, but I don’t have a reason to.
One of my original ideas to use the GUI for replications was a “Syncoid” style, simple backup: with a single click it will create a new snapshot (i.e, @backup-YYMMDD), and send it incrementally to a backup pool. (No need to tether a Periodic Snapshot Task or “naming schema” of a separate snapshot pattern to the replication job.)
This is not possible with the GUI.