Because it's insanely easy to use and makes beautiful animations, that's why. There are other physics-based animation systems out there (e.g. UIKit's spring animations, Facebook's Pop), but they still require writing too much code, bookkeeping, and hand-holding for our taste. YapAnimator represents a distilled n-th iteration of code that we've been using in our own apps for years. We find it invaluable in our day-to-day and think that you will too. YapAnimator comes with a handy extension bolted on to CALayer and UIView/NSView, providing one-liner animations under the animatedLayer and animated properties, respectively.