What is the main purpose of constructor?

1.  To initialize local state by assigning object to this.state
2.  For binding event handler methods to the instance
    For example, the below code covers both the above cases,

    constructor(props) {
      super(props);
      // Don't call this.setState() here!
      this.state = { counter: 0 };
      this.handleClick = this.handleClick.bind(this);
    }