Animations should be used to improve the user experience of your application. They shouldn't be used just to delight the user as these tend to end up being pointless and actually hinder your users.
Animations can be used to reduce cognitive load, define relationships between elements, provide visual hints, and much more.
The focus of this book is going to be teaching you the Animated library inside of React Native. If you need a reason to animate there are a TON of studies/posts/research on animations that serve a purpose.
Overall animations are generally added later but provide immense value to your users and I will teach you how to breakdown animations, understand them, rebuild them, and implement them in your own applications.
Below I will link to some great resources I enjoyed reading on developing animations that are useful.
Meaningful Motion with Action Driven Animation - This one is one of my favorites, subtle techniques that add a lot of value.
How Functional Animation Helps Improve User Experience
Experience Design Essentials: Animated Micro interactions In Mobile Apps
How To Design Error States For Mobile Apps
Also here are some resources to get some inspiration for animations. Don't forget to look at your favorite applications and see what subtle and functional animations they are using.