$_SERVER['DOCUMENT_ROOT'] is incredibly useful especially when working in your development environment. If you're working on large projects you'll likely be including a large number of files into your pages. For example:
<?php
define("REGISTRY_CLASSES", $_SERVER['DOCUMENT_ROOT']."/SOAP/classes/");
define("REGISTRY_CONTROLS", $_SERVER['DOCUMENT_ROOT']."/SOAP/controls/");
define("STRING_BUILDER", REGISTRY_CLASSES. "stringbuilder.php");
define("SESSION_MANAGER", REGISTRY_CLASSES. "sessionmanager.php");
define("STANDARD_CONTROLS", REGISTRY_CONTROLS."standardcontrols.php");
?>
In development environments, you're rarely working with your root folder, especially if you're running PHP locally on your box and using DOCUMENT_ROOT is a great way to maintain URL conformity. This will save you hours of work preparing your application for deployment from your box to a production server (not to mention save you the headache of include path failures).
