Why have to bind this in router function?

[krave1986]

I am researching some example code about React based on director.js.
The example code shows that I have to bind thisArg to make sure the app could work well.
Could you please tell me why I have to bind this in router function?
Here is the snippet of the code.

        componentDidMount: function () {
            var setState = this.setState;
            var router = Router({
                '/': setState.bind(this, {nowShowing: app.ALL_TODOS}),
                '/active': setState.bind(this, {nowShowing: app.ACTIVE_TODOS}),
                '/completed': setState.bind(this, {nowShowing: app.COMPLETED_TODOS})
            });
            router.init('/');
        },