The Clipboard is a temporary storage area used by Vista. Information (such as text, files, graphics, sound, or video) can be copied to the Clipboard from one program or location and pasted elsewhere. The Clipboard can only hold one piece of information at a time. Whenever something is copied to the Clipboard, it replaces whatever was there before automatically.
This trick can be handy if you just want to clear the Clipboard without having to copy something else to do so. This way you do not have potential sensitive information just sitting in the Clipboard.