There are many reasons why you should use a caching plugin. One of them is that it saves resources, by not going to the server every time a new page is requested. Instead, it takes it out of already existing cache.

There are many plugins for WordPress that do caching. However, we recommend the one below:

Plugin Name: WP Super Cache
Plugin Link: http://wordpress.org/extend/plugins/wp-super-cache/
FAQ: http://wordpress.org/extend/plugins/wp-super-cache/faq/
Installation: http://wordpress.org/extend/plugins/wp-super-cache/installation/