This reports the IP address of an outgoing interface used by the responding router to forward the queried multicast stream, per its description, part of a list of such interfaces returned by the mrbranch trace. It shows an admin exactly which of a router's interfaces is actively forwarding a given multicast group's traffic, which matters when a router has several candidate outgoing interfaces and only one should be active per the current tree. Seeing an unexpected interface address here rather than the one an admin intended for that multicast group would reveal a routing or IGMP-join misconfiguration.