A web application that is providing HTTP API for outsiders for performing a processing task and you want to give the results for the caller as a response to the HTTP request they made.
A request comes in, message describing the task is pushed to queue by the frontend server. After that the frontend server blocks to wait for response message with the same correlation id. A pool of worker machines are listening on queue and one of them picks up the task, performs it and returns the result as message. Once a message with right correlation id comes in, frontend server continues to return the response to the caller.