Summarizing a book into a report speaks volumes. It tells a teacher and a parent (if they are paying keen attention) some cool things about their kids. It shows the kid's ability to comprehend stuff: story or material that they came across ( via a book, movie, games, plays, libraries, school, fun time etc ), It shows their understanding of emotions, some fun facts they read, characters they came across and relate to. They try to figure out what the author is trying to convey/where the story is heading.
When they are asked to write a book report (which is part of school curriculum) by the teacher or a parent, they will need to articulate what they read, summarize in a nutshell facts in the book, characters in the story line, and importantly what they thought about the book, and why and what they found interesting.
Teachers can also gain a insight into kids likes & dislikes, how much they understood the story/material, how the kids creative writing is developing.
-- book information
-- title, author
-- when and where the story takes place,
-- who are the main characters
-- what was the main problem
-- what was the solution
-- How does the kid like the book