Besides mapping URLs to actual servlets, you can use the DD to customize other aspects of your web application including security roles, error pages, tag libraries, initial configuration information, and if it’s a full J2EE server, you can even declare that you’ll be accessing specific enterprise javabeans.
Don’t worry about the details yet. The crucial point for now is that the DD gives you a way to declaratively modify your application without changing source code!
Think about this... it means that even those who aren’t Java programmers can customize your Java web application without having to drag you back from your tropical vacation.
The deployment descriptor (DD), provides a “declarative” mechanism for customizing your web applications without touching source code!