Every element on a page fights for the attention of the user. Text sits next to form elements, which sit next to ad banners, which sit next to logos, taglines, copyright information, and persistent navigation. How is a user supposed to know which things really need attention and which things can safely be ignored?
Interfaces full of unnecessary graphical elements, text that's longer than it needs to be, and features that don't really help anyone risk creating an environment of visual overload. Every time a user accesses a page, his brain must take a moment to process everything on the screen to see what's there and analyze the importance of each element. This moment can be disorienting when a page is jammed full of content and graphics.
Clutter diminishes a user's ability to form a workable mental model by crowding the important pieces of a screen in with unimportant ones. Clutter makes it more difficult for new users to become intermediate users by putting things in the way of the learning process. Clutter makes it hard to see the design in the design.
Clutter makes a mess of things.
Cluttered interfaces result in comments like:
"It's as though the marketing department threw up on this page."