Why do books have a glossary?

books have glossaries because if you dont understand a Word, or if its a textbook, and the teacher gives you an assignment, 1-25 with the vocabulary Words. you dont have a dictionary and the only other thing is the glossary in the back of the book. that will probabally be the most helpful thing you ever use in your entire school career. its in your science textbook, you social studies book, even your math book.