This read-only PhysAddress is the MAC destination address recorded in the Layer 2 forwarding information used when IP-routing a packet at Layer 2. An admin cares because this is the actual address hardware writes into the outgoing frame, so mismatches here directly explain frames arriving at the wrong host or being dropped by the intended recipient. For example, if a server stops receiving forwarded traffic after a NIC swap, this field would still show the old MAC until the L2 info entry is refreshed.