I think it's literally just the equivalent of an envelope - a "to" field and a "from" field, so that any messenger in charge of deliveries can, at a glance, tell to whom this message is meant to be delivered, and to return it to the sender if it's undeliverable.

I wonder if ancient Ur had dodgy FedEx delivery guy equivalents who would knock on your door and toss the tablet in the dirt and mark it as "delivered".