node.js front end web server

I have seen lots of posts where people are recommending nginx or something similar as the front end web server for node.js.
Since node has the ability to create a web server, such as:

var http = require('http');
var static = require('node-static');
var file = new static.Server();
var url = require('url');

var index = require('./serverJS/index.js');
var login = require('./serverJS/login.js');
var admin_index = require('./serverJS/admin_index.js');
var admin_login = require('./serverJS/admin_login.js');

http.createServer(function (req, res) {
    if (url.parse(req.url).pathname == '/index') {
        index.serve(req, res);
    } else if (url.parse(req.url).pathname == '/login') {
        login.serve(req, res);
    } else if (url.parse(req.url).pathname == '/admin/index') {
        admin_index.serve(req, res);
    } else if (url.parse(req.url).pathname == '/admin/login') {
        admin_login.serve(req, res);
    } else {
        file.serve(req, res);
    }
}).listen(9000 , '127.0.0.1' );

Q: In what case scenario would you need another web server in adition to node's?