Based on the article outlined in the React docs: You Might Not Need An Effect, they explain that logic which should only be executed once should be wrapped in an init clause.

i.e.

// Will run init flow only once per application lifecycle

let didInit = false;

function App() {
  useEffect(() => {
    if (!didInit) {
      didInit = true;

      loadDataFromLocalStorage();
      checkAuthToken();
    }
  }, []);
}

Or:

// Will run init flow whenever component is mounted

function App() {
  const didInit = useRef(false);

  useEffect(() => {
    if (!didInit.current) {
      didInit.current = true;

      loadDataFromLocalStorage();
      checkAuthToken();
    }
  }, []);
}

However, if moved outside the effect, the behavior remains the same. So if the behavior’s the same, why would you need the hook?