¿Por qué el custom hook useFetch usa el generic <T,>?

const useFetch = <T,>(url: string) => {
  const [data, setData] = useState<T | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<Error | null>(null);

  useEffect(() => {
    fetch(url)
      .then((res) => res.json())
      .then((json: T) => setData(json))
      .catch((err) => setError(err))
      .finally(() => setLoading(false));
  }, [url]);

  return { data, loading, error };
};

const { data } = useFetch<User[]>("/api/users");