The purpose of a clean desk

I listened to one of the recent 43Folders conversations with David Allen (can't remember which one). Merlin Mann noted, during one of his conversations, that David Allen always has a clean desk and only the materials relevant to the task on hand on his desk. I think that's a good idea. It reinforces one of the principles of GTD which is to concentrate on one task and only that one task at a time. I will try to keep my desk clean more often.