A、To know whether they read the book or whether they understand the book. B、As a common home assignment. C、To help improve their

Book reports may be required by teachers who want to know whether their students have read the books assigned and how well they understand those books. If they are not required, students may as well write one or two for themselves, for writing book reports helps students to improve their reading comprehension and their ability to analyze and evaluate books. Moreover, it is very good writing practice.
Generally speaking, a book report consists of the following three main parts: information about the author and his times, a summary of the book, and comments on it.
A brief account of the author’s life should be given together with a description of his times. The latter should include the circumstances that led to the writing of the book under discussion and the historical and social background related to the content of the book.
As has been mentioned in the previous section, a summary should be self-contained, clear, and easy to understand. Above all, it should be objective. Comments on and criticisms of the book should not be mixed with the summary; they can be left to the third part.
Comments on the evaluation of the book form the third and most important part of a book report. In this part the writer expresses his or her own views on the book, names its merits and demerits, and discusses its relevance to the present time.
Question: What does this passage mainly discuss?

Why do teachers require their students to write a book report?