Add an interface on your site to write a post (or page, or anything), without having to go into the admin section. Also allows for ‘anonymous’ posting (not logged in users, still asks for name/email) with a recaptcha. This makes Post From Site a perfect plugin for your user reviews, a suggestion box, or even a very basic forum site.
After install, you can display a form on your site via a widget, shortcode, or PHP code in your theme. See this page for further documentation.