What is the usage of "server_app"?

[kigawas]

class PyInstrumentProfilerMiddleware:
    def __init__(
        self,
        app: ASGIApp,
        *,
        server_app: Optional[Router] = None,
        profiler_interval: float = 0.0001,
        profiler_output_type: str = "text",
        is_print_each_request: bool = True,
        **profiler_kwargs,
    ):
        self.app = app
        self._profiler = Profiler(interval=profiler_interval)
        self._server_app = server_app

It looks like the server_app is used nowhere, can you explain what it is?

[sunhailin-Leo]

@kigawas add_evevnt_handler