From the article:
We often hear about the benefits of reading storybooks at bedtime for promoting vocabulary, early literacy skills, and a good relationship with your child. But the experts haven’t been in your home, and your child requests the same book every single night, sometimes multiple times a night. You both know all the words off by heart.
Given activities occurring just before sleep are particularly well-remembered by young children, you might wonder if all this repetition is beneficial. The answer is yes. Your child is showing they enjoy this story, but also that they are still learning from the pictures, words, and the interactions you have as you read this book together.