Its funny a similar issue should come up so close together.
See Error attempting to attach LXD VM NIC to host's physical network interface - #2 by tomp for a full explanation.
But basically the idea of adding support for using a physical network with a physical NIC was for consistency rather than being useful in most common scenarios.
When using a physical NIC (Linux Containers - LXD - Has been moved to Canonical) the actual host interface is moved into the instance so it no longer appears on the host.
This means that when using a physical network for an instance, it can only be used with a single running instance at a time. This maybe useful in certain niche circumstances.
But when we were discussing this with @ru-fu we did add some notes to the physical NIC documentation (Linux Containers - LXD - Has been moved to Canonical):
A
physicalNIC provides straight physical device pass-through from the host. The targeted device will vanish from the host and appear in the instance (which means that you can have only onephysicalNIC for each targeted device).