This allows running multiple services side-by-side, and clearly tell each one which interfaces they shall manage. Of course they might still fight for the devices that are not tagged like this, but at least in the networkd case we should be safe, since by default it will not try to manage an devices it has no configuration for.
In systemd v256 we now made it possible to configure this udev property (or any other in fact) via the Property= field in .link files.