Interesting, but a bit misleading. I was expecting something more like HotJava, where the entire thing is java top to bottom. A webkit wrapper for Selenium tests is definitely useful, just not as neat as the idea of a pure java browser.
It uses only Java SE. It implements the Selenium API and wraps the renderer and JS engine that's built in to Java (WebKit). Granted, Java itself is built on native code and native WebKit renderer (whereas, the networking code, graphics, cookie management, headers, etc. are Java). Sorry to be misleading.
Glad I'm not alone. Obviously it can download pages like wget or curl. Presumably you could access the DOM of a page - for scraping, etc. Then there is a testing - perhaps this would be used for automated access of your website to make sure its working. But these are all guesses.
The primary difference from wget and curl is the ability to fully render pages, sub-resources, CSS, and JS. Also allows interaction such as clicks and keyboard events.