Go's http.MaxBytesReader, why pass in writer?

Intuitively, I would think that when you create a MaxByteReader and pass in the http.ResponseWriter, it would write out the status code for you. But that isn't the case, what does the writer actually do?

example:

func maxBytesMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        r.Body = http.MaxBytesReader(w, r.Body, 1)

        next.ServeHTTP(w, r)
    })
}

func mainHandler(w http.ResponseWriter, r *http.Request) {
    var i interface{}
    err := json.NewDecoder(r.Body).Decode(&i)
    if err != nil {
        fmt.Println(err.Error())
    }
}

func TestMaxBytesMiddleware(t *testing.T) {
    handlerToTest := maxBytesMiddleware(http.HandlerFunc(mainHandler))

    req := httptest.NewRequest(http.MethodPost, "http://test.com", bytes.NewReader(json.RawMessage(`{"hello":"world"}`)))

    recorder := httptest.NewRecorder()
    handlerToTest.ServeHTTP(recorder, req)

    if recorder.Result().StatusCode != http.StatusRequestEntityTooLarge {
        t.Errorf("expected %d got %d", http.StatusRequestEntityTooLarge, recorder.Result().StatusCode)
    }
}

but when this test runs I get this:

http: request body too large
--- FAIL: TestMaxBytesMiddleware (0.00s)
    main_test.go:37: expected 413 got 200

if I want the desired functionality of what I thought this function did, I need to change my mainHandler to something like this:

func mainHandler(w http.ResponseWriter, r *http.Request) {
    var i interface{}
    err := json.NewDecoder(r.Body).Decode(&i)
    if err != nil {
        if err.Error() == "http: request body too large" {
            w.WriteHeader(http.StatusRequestEntityTooLarge)
            return
        }

        fmt.Println(err.Error())
    }
}

So what is that writer even there for?