Cos the more you surf and the more content you look and and information you download (every time you view a webpage you download info, we're not just talking about media downloads), the more your computer fills up with temp files and cookies etc.

You have folders that store temp files - script, jpegs/gif, ini files etc - these are to help windows when you revisit web pages - but the more you fill up, the more bogged down you machine will become.