The first good reason, quoted by everyone, is that a tidy desk helps you find what you need in order to complete the tasks in hand.
A second reason given has something to do with "an organized desk equals an organized mind."
That second reason may be true, but it doesn't tell the whole story.
The thing about being surrounded by clutter and piles of papers is that it signals to your brain that there are a dozen other things to do.
While you are trying to focus on the job in hand, a part of your subconscious mind is devoting itself to every other incomplete task you have on your plate, both personal and business.
This is why dealing with the paper on your desk needs to be part of your productivity planning.
Once every piece of paper is in its proper place, your mind can relax and focus just on the task in hand.
Some of that paper needs to go into the trash. Some of it needs to be filed. Some needs to be out in your "to-do" tray.
It's only when everything is in its place that your mind can relax in the knowledge that it now need focus only on the job you are working on right now.