I don't think you get the point of this app or the check box. This is for an app that is meant to foster habits and the checkbox animation is an infrequent reward for completing a habit forming task. Secondarily the goal was also to win over the Apple user demographic and win a design award.
You're thinking of user interface in an app that you have some intrinsic motivation to use, and the UI in that case is just a means to an end.