The cache plays an important role in keeping little pieces of info from sites you visit often on your local machine, making the site load faster since it only has to load new information.
For example, when you visited my blog your browser downloaded the graphics associated with my theme. The next time you visit (and I hope you will) your computer will already have this info and won’t have to download it again, saving your bandwidth and loading the site quicker.
If you want to clean up every once and a while, follow the steps below.
On Mac this is the shortcut: Shift+Command+Delete.
On Windows, it is: Ctrl+Shift+Delete.