When you are using your computer for day to day activities, it collects all sorts of information about where you go on the internet, what pages you open, and which web sites you visit.
All the details on forms you fill up. All other programs are storing information about the recent documets and files you opened and so on. Not only this garbage is cluttering your hard disk, but it can represent danger to you if it is discovered by your family or your manager at work. To protect yourself, use Evidence Destructor.