What benefit does it offer over systemd-networkd?

It's a way to use systemd-networkd without having to write network and netdev unit files. You write YAML instead and it gets rendered to the aforementioned unit files. Some people will prefer that, and some people will hate that. :)

I don’t hate it, but I don’t see that much use in the extra layer of abstraction. The docs aren’t that great either

It is an abstraction layer over systemd-networkd (server) and NetworkManager (desktop).

It is a self-inflicted problem where a solution is another layer of abstraction. Red Hat ships NetworkManager on server too, so they have unified configuration without another layer.