why does listing the actual constructor of a class in javascript important

I was reading on javascript garden http://bonsaiden.github.com/JavaScript-Garden/ about prototype in javascript and one of its example goes like this:

function Foo() {
    this.value = 42;
}
Foo.prototype = {
    method: function() {}
};

function Bar() {}

// Set Bar's prototype to a new instance of Foo
Bar.prototype = new Foo();
Bar.prototype.foo = 'Hello World';

// Make sure to list Bar as the actual constructor <-------------------
Bar.prototype.constructor = Bar;

Notice the line that reads Make sure to list Bar as the actual constructor. I really am lost about what this does/is. I have tried making new instances of Bar() with and without the last line. But call "value" or "method" on those instances return the exact same things. So I wonder, what's the need (I assume there must be one) of specifying the constructor?

Thank You!!!