PhantomJS 1.3 example: prevent window.onload

prevent-onload.js

var page = new WebPage(),
    address;

// Let's see what the web page's console might output.
page.onConsoleMessage = function (msg) {
    console.log('from web page: ' + msg);
};

// Hijack assignment to window.onload to do nothing.
page.onInitialized = function () {
    page.evaluate(function () {
        window.__defineSetter__('onload', function (handler) {
        });
    });
};

if (phantom.args.length === 0) {
    console.log('Usage: prevent-onload.js <some URL>');
    phantom.exit();
} else {
    address = phantom.args[0];
    page.open(address, function (status) {
        if (status !== 'success') {
            console.log('FAIL to load the address');
        } else {
            console.log('Loading completed.');
        }
        phantom.exit();
    });
}

i have a question is what is the use of Hijack assignment to window.onload?What can we do?