Nice Standards Article explaining the why....
When companies create the software behind their Web sites and Web applications, they often fail to develop a proper architecture for the user interface (UI) layer. Many in the software and Web development industries consider the UI layer pitfalls from the 1990s as the norm; they do not realize that a more progressive, structured, and thought-out UI layer is not only possible, but will interact much more effectively with backend software. Simplified connections between the UI and backend software layers are one of the primary benefits of a properly-architected UI layer—one developed using Web standards-based techniques that separate content and structure from presentation and behavior. This article will first introduce persistent UI pitfalls of the 1990s, then outline the benefits of Web standards-based Web site development.
Web Standards Article