As opposed to the classroom model of: [lecture -> assign work -> grade & return with static feedback], students can actually play with the subject matter during the lesson, instead of turning pens into projectiles or doodling in their books because they're bored to death (not that I know anything about that). Then they get immediate feedback, whether right or wrong, by seeing how the instructor would have solved the problem. That kind of hands on learning, where the student learns through their own trials and errors is much more fun than sitting through a lecture that has to accomodate the varied learning paces of a classroom of 20+ students.
The video posted here unfortunately focuses on the live editing aspect of the app, but you can see the interactive lesson function more clearly in this early prototype video[1].
This kind of interactive learning is the same thing that Sebastian Thrun is working on at Udacity[2]. For all the promise of making university courses available for free online, I think this is the truly disruptive stuff going on in online education, because it's way beyond just filming lectures and throwing them online, it's a fundamental leap forward for education.
[1] http://www.youtube.com/watch?v=nvaaude_1hk [2] http://www.youtube.com/watch?v=75TP3hoPA8U