Community-driven effort of third-party action hooks. This appearance-focused hooks kinda remind me to Genesis hooks 1 which is highly usable when building a child theme. IMO, hooks which allow developer to easily extend anything imaginable is one of the most awesome thing on WordPress. However, it adds another layer of complexity. The learning curve become steeper but once you got it, it becomes very-very useful.
As far as I can remember, Genesis has tons of action hook for child theming purpose ↩