JS function without brackets?

Hey everyone,
Got a JS syntax question regarding arrow functions. Ill post the code first so the question will be easier to understand:

render() {
      return (
        <button className="square" 
                onClick={() => this.setState({value: 'X'})}>

This is a code fragment from a React app. The onClick handler calls an arrow function to this.setState.

Why isn’t this.setState in curly brackets like a normal function? What are some other use cases for this syntax?

Thank you in advance!

Plenty of resources explain the syntax.

Thanks. These were helpful.