The Briefs toolkit was created for rapidly building & iterating app prototypes for the iPhone. These prototypes run directly on the phone, like actual apps, but require much less time and code to produce. Less code means faster cycles and cheaper development.
The purpose of Briefs is to create better apps that have a tested interaction design before they are built. Unlike traditional mockups, briefs are used just like an actual app not just seen. And because they use the same frameworks that regular apps are built upon, briefs can potentially take advantage of animation, device rotation and gestures. You can even design the experience around app interruption due to an incoming phone call. Thinking about these interactions will ensure a more thought out experience for your users.