Ever watched youself browsing e.g. a web forum? Noticed that you viewed the same page several times? Well, this means extraordinary and useless load for your server if there's no caching mechanism implemented in the web application. Even if there is some file or db cache you can still improve performance with implementing some http cache.
With a few simple lines of code using HTTP_Header, you can tell your scripts how long to set the "expires" header to on your requests. This increment (in seconds) is relayed to the browser to tell it when to next fetch the page and not reload from cache.