Let's say you have a todo app with a million users checking an average of five boxes per day. Currently, you have a simple color shift animation which lasts 200ms and doesn't prevent the user from interacting with other elements. Then a designer suggests a fancy animation popover which takes 3s and blocks interaction, because they read some article about gamification and how it'll increase user engagement.

Disregarding that your app is about providing a specific function and user engagement metrics are meaningless, you implement the fancy animation. Your users are now collectively spending 1,400,000 hours per year waiting for an animation to finish.

Please don't be that person. Your users' time is more valuable than your desire for UI whimsy.