Currently, we need to use the subclass like AnimatableView. If we can provide an extension property like ib to UIKit components, then we can use the properties from the protocols e.g. Animatable, CornerRadiusDesignable etc. Then we can use this ib property to set the animatable or designable properties like view.ib.cornerRadius = 20 in code. view is a just pure UIView not AnimatableView.

Since IBAnimatable has some powerful protocols, some developer they don't want to use the subclasses (e.g. AnimatableView) and they don't want to make a subclass by themselves. Providing an extension property ib can help them use IBAnimatable easily.