Imagine, you allow users (let's call them Authors) to submit articles at your web-site. Your managers review such an article and publish it. But some day later the Author is stolen by aliens/ZOG/international terrorists or other good people. So he can change the alredy published article at your web-site to place a swastika there. Just use this plugin and what has been published cannot be changed by later except by managers with publishing permissions.
Just install and publish the plugin.