Why must all EK9 service methods return HTTPResponse?

defines module qa.webservicesdeep.service.return.httpresponse

  defines service

    HealthEndpoint :/health open

      healthCheck() as GET for :/status
        <- response as HTTPResponse: () with trait HTTPResponse
          override content()
            <- rtn as String: `{"status": "healthy"}`
          override status() as pure
            <- rtn as Integer: 200
          override contentType() as pure
            <- rtn as String: "application/json"
          override cacheControl() as pure
            <- rtn as String: "no-cache"
          override contentLanguage() as pure
            <- rtn as String: "en"
          default operator ?

  defines application

    HealthApp
      register HealthEndpoint()

  defines program

    ServiceReturnDemo()
      stdout <- Stdout()
      stdout.println("Service methods must return HTTPResponse")