Why does this JavaScript code (module pattern for RequireJS and Node.js) work?

With my limited understanding of RequireJS and Node.js (plus JavaScript in general), I usually take a look at the source of some well-known JavaScript libraries. Every time I see something like this:

( // Wrapping
    function (root, factory) {
        if (typeof exports === 'object') { // Node.js

            var underscore = require('underscore');
            var backbone = require('backbone');

            module.exports = factory(underscore, backbone);

        } else if (typeof define === 'function' && define.amd) { // Require.JS

            define(['underscore', 'backbone'], factory);

        } 
    }(this, function (_, Backbone) { // Factory function, the implementation
        "option strict";

        function Foo() {}

        return Foo; // Export the constructor
    })
); // Wrapping

What I can understand (hopefully):

• The anonymous function that wraps the code is automatically executed when the script is uncluded in a <script> tag
• This code works with both RequireJS and Node.js (if checks in the very beginning); the result of factory function is either assigned to module.exports (Node.js) or used as argument of define function (RequireJS).

Q1: how this code works without RequireJS and Node.js? if and else if checks would fail, factory function is never executed and the scripts returns nothig.

Q2: what's the purpose of passing this as root argument? It's never used