Initially, Quartz Composer is a “visual programming” environment to create animated scenes.
But recently, some designers have started to use it to mockup animations and interactions.
Because the primary goal of Quartz Composer is to create motion graphics, it makes sense to use it for interaction prototyping. The problem is that there isn't a lot of patches (a patch is like an instance of a class in a classic object-oriented programming language) dedicated to user interface in Quartz Composer.
That's where Origami steps in:
Origami provides a set of tools for Quartz Composer that make interaction prototyping a lot easier.
Animations, transitions, buttons, layers, phones, etc.
I recommend to watch this video to get a brief overview of what is possible with Origami. It's intriguing, it looks promising and it motivates me to learn Quartz Composer more seriously.