The destination-host AVP is cleared in all Diameter request messages that are retransmitted by the Diameter application (NASREQ, Gx, Gy). Messages retransmitted by the Diameter Base (peer failover related retransmissions) do not have the destination-host AVP cleared.
The Diameter application retransmits the message with the assumption that the path to the destination-host, or the destination-host itself is unavailable. Clearing of the destination-host AVP provides a new chance to for the Diameter Base to deliver the message to an alternate destination in the same realm. This alternate destination must be in a geo-redundant setup where sessions are synchronized between the geo-redundant nodes.