It's a simple service on the surface. You hand it a URL, it gives you back a thumbnail of that web site. At first glance you may think..."ok, why?" The answer to that is of course, "it depends".
He gives examples of reasons it would be needed (designers wanting to show off screenshots of a site or a replacement for Alexa's thumbnailing service) and the code that is needed to make things work. The script asks for the three properties the API needs - a URL for the target site, the height of the browser "window" taking the shot, and the width of this same "window".
Cal show examples of the messages sent back and forth and includes the code for the PHP 5 class to make the request along with some sample usage code. The class makes a basic HTTP request to the API with the XML payload and waits for the response - an estimate of the time for the job and the URL that the results can be found at.