This scenario is largely the same as the API Transformation scenario from an installation perspective. The only difference is that the requests in this scenario comes from end users rather than server applications and the target for the requests are existing web applications or Software as a Service applications.
This scenario lends itself to a myriad of use cases:
Digital transformation, where an existing application (often beyond the control of the business) is functionally enhanced, without the explicit need for the existing application being aware of these changes
Bot management, where the X Engine detects bots and adds policies for how those bots are able to access the underlying application
Robotic process automation where requests to the existing application results in data also being entered into secondary systems
Orchestration of multi existing applications that are joined together to form a new experience
… and many more
This scenario is especially useful for regaining control of web applications that are otherwise difficult, expensive or impossible to change