Zend loader is much similar to require_once, best usage is if we have to load a file based on the variable value.

<?php

require_once "Zend/Loader.php";

Zend_Loader::loadClass('Zend_Session');

Zend_Session::start();
?>

The above example, illustrates loading the zend session class and starting the session.

Here i am not going to discuss complete usage of zend loader, i will be explaining zend framework concepts to quick start with project.

Reference link:
http://framework.zend.com/manual/en/zend.loader.html