Probably something you want to have to get your content put out there faster. Doesn’t hurt, right? And there’s a plugin if you’re hosting WP yourself.
In a nutshell, instead of having to periodically ask your blog if there are any updates they can now register to automatically receive updates each time you publish new content. In most cases these updates are sent out with in a second or two of when you hit the publish button.
via a Matt Cutts tweet.