What is useEffectEvent for, and why can't you call an Effect Event during render?

function ChatRoom({ roomId, theme }) {
  useEffect(() => {
    const connection = createConnection(roomId);
    connection.on("connected", () => {
      showNotification("Connected!", theme);
    });
    connection.connect();
    return () => connection.disconnect();
  }, [roomId, theme]); // Changing the theme reconnects the chat.
}