Why would you want to clear your cache?
First of all, let me explain what 'cache' is. Traditionally, cache is a surplus of anything. An excess of something. When you visit a website, you're downloading that website (code, images, text, etc.) to your computer in the form of cache or temporary internet files. Clearing your cache means removing those files or website from your computer.
Why would you want to do this occasionally?
a.) it ensures you're looking at the most recent version of that website, and
b.) it helps your browser run more efficiently.