Returning by reference

[logan]

class Foo
{
	int data;

public:
	Foo (int n):data (n)
	{
	}

	Foo ():data (0)
	{		
	}

	Foo &operator= (int n)
	{
		data = n;
		return *this;
	}

	Foo &operator= (const Foo& foo)
	{
		data = foo.data;
		return *this;
	}

	void display ()
	{
		cout << "data = " << data << endl;
	}
};

Why in the above code the operator= returns by reference? If I return it by value (Foo operator= ()) it seems to work fine. Do we return by refernce because it is faster (only the address of the object will be returned instead of making a copy of the object and then returning it, please correct me if I'm wrong )?

Thanks.