What is the purpose of using super constructor with props argument?

A child class constructor cannot make use of this reference until the super() method has been called. The same applies to ES6 sub-classes as well. The main reason for passing props parameter to super() call is to access this.props in your child constructors.

Passing props:

    class MyComponent extends React.Component {
      constructor(props) {
        super(props);

        console.log(this.props); // prints { name: 'John', age: 42 }
      }
    }
    

Not passing props:

    class MyComponent extends React.Component {
      constructor(props) {
        super();

        console.log(this.props); // prints undefined

        // but props parameter is still available
        console.log(props); // prints { name: 'John', age: 42 }
      }

      render() {
        // no difference outside constructor
        console.log(this.props); // prints { name: 'John', age: 42 }
      }
    }
    

The above code snippets reveals that this.props is different only within the constructor. It would be the same outside the constructor.