This is commonly used with transports, such as HTTP, that do not allow duplex communications natively. TCP, by contrast, does allow duplex communications natively and so does not require the use of this binding element for the service to send messages back to a client.
The client must expose an address at which the service can contact it to establish a connection from the service to the client. This client address is provided by the ClientBaseAddress property. Note that Windows Communication Foundation (WCF) auto-generates a ClientBaseAddress if one is not explicitly set by the user.