End material is typically comprised of essays or editorials on the book itself, and is not commonly seen outside of academic “study editions” of certain very famous books, such as John Steinbeck's The Grapes of Wrath.
As with most front material, all end material is totally optional.
If you greatly enjoyed a book, end material can give you a chance to revisit parts of it; if you didn't understand the importance of a book, it can provide important historical and cultural context. Otherwise, most people ignore it.