When do I use setState in Flutter?

According to the docs:

Calling setState notifies the framework that the internal state of this object has changed in a way that might impact the user interface in this subtree, which causes the framework to schedule a build for this State object.

So if the state of the widget changes you have to call setState to trigger a rebuild of the view and see immediatly the changes implied by the new state.

Anyhow the below snippets are equivalent.

first case (directly form flutter create <myproject>):

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {

    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

second case:

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    _counter++;
    setState(() {});
  }

What I don’t know is the reason why and if the first case is the conventional way to use setState, I would say because of readability of code.