Why YUI.lang.extend implements Inheritance this way?

Question

I'm reading YUI2.8.1 source code yahoo/yahoo.js.

The YAHOO.lang.extend method is implemented this way http://github.com/yui/yui2/blob/master/build/yahoo/yahoo.js

I don't understand why it creates another F function. IMHO, below code should also work(ignoring overrides part)

function extend (subc, superc ) {
    if (!superc||!subc) {
          throw new Error("extend failed, please check that " +
                    "all dependencies are included.");
    }
    subc.prototype=new superc();
    subc.prototype.constructor=subc;
    subc.superclass=superc.prototype;
}

There must be reason that YUI implements it this way, but I cannot figure it out. Anybody can shed some light on this?

Thanks