Your computer's desktop is much like a physical desktop. You probably keep a number of commonly used items on your desk such as pens, papers, folders, and other items. Your computer's desktop serves the same purpose -- to give you easy access to items on your hard drive. It is common to store frequently used files, folders, and programs on your desktop. This allows you to access the items quickly instead of digging through the directories on your hard drive each time you want to open them.
Both the Macintosh and Windows interfaces use the desktop as a central part of the interface. Both operating systems allow you to move items on and off the desktop as you wish and offer organization tools to arrange and clean up the items on the desktop. Yes, it would be nice if there was an option like that for a real-life desktop. You can also customize your computer's desktop with the pattern or background image of your choice. For more information on customizing your desktop, view this Help Center article.