Javascript function without any code in it

Question

I found the following javascript code snippet as part of a popup window, which is causing my browser to ask if I really want to close the window.

window.onbeforeunload = function() {
    setTimeout(function() {
        window.onbeforeunload = function() {};
        setTimeout(function() {
            document.location.href = 'index.html';
        }, 500);
    },5);
    return '';
};

I am wondering about these things:

• why would you use a function without any code in it ( code line 3 )
• why do you use window.onbeforeunload inside itself a second time?

Or does this just try to obfuscate its purpose ?