https://docs.mongodb.org/manual/tutorial/deploy-replica-set/#architecture

In a production, deploy each member of the replica set to its own machine and if possible bind to the standard MongoDB port of 27017. Use the bind_ip option to ensure that MongoDB listens for connections from applications on configured addresses.

This conflates the meaning of "binding" to a network interface and "listening" on a port.

This is important because when setting up a replica set on separate physical machines, it is required that MongoDB servers listen to an externally accessible network interface.