Why does the cover of a book have to grow partially

The cover grows out of a part of the so-called edge protection, is used to protect the book, bumping and bumping is not so damaged inside the paper pages;

title page:

book production, before and after the two pages shall not be glued, in the pamphlet, often with translucent or other special paper to do the title page, through the link between it and the traditional bookbinding, to enhance the publication's elegance, the title page also refers to a page in the book cover or lining page, before the main text, the title page is generally printed with the title of the book, author or translator name, publisher and month of publication, etc., the title page also plays a decorative role or liner notes, a page before the text, the title page is generally printed with the title of the book, the name of the author or translator, the publisher and the year and month of publication, the title page also plays a decorative role in increasing the beauty of the book;

The title page in the book and the book is essential to the two pages, and at the same time, the title page in the book as well as the role of the book and the book and not exactly the same.