The options that it allows you to animate the design prototypes straight from the sketch. The smoothness that the animation comes out. The ability to export in .mov and .gifs straight out of the Principle Preview. The different easing options that the animation builder allows you to use.

The fact that you can do so much in such a quick period of time

This is the best software for any designer or UI Developer to begin with. For somebody who hasn't given much time in Flash or other time frame based tools.