In this example we have an existing BizTalk infrastructure on premise and there are some existing messaging or orchestration processes implemented. In addition to the normal use of these processes we want to expose BizTalk to the cloud. In BizTalk you can use the WCF adapters with the WCF relay bindings which allow you to use BizTalk as the listening service on the relay endpoint.
This means that when the client application (in the below case CRM) makes a call to the service, the message is routed into BizTalk and further processing can take place.
In this architecture the value provided by the Azure Service Bus Relay is that it gives you a simple way to bridge BzTalk out to the cloud. The value provided by BizTalk is that it can reuse existing services or processes in BizTalk and that BizTalk gives you a reliable and durable platform where messages can be processed asynchronously from the original caller of the service.