When I was young, books always had "The End" at, well, the end. Maybe publishers back then thought we wouldn't know the book was over without the words. While conventions have changed, it is still satisfying as an author to write the words at the end of a project, even if it's just a rough draft. Endings are a goal post you need to cross, even if you ultimately decide that first book is a learning experience and not going to be published. Knowing you can finish gives you a psychological edge with the next book. It really does help to know you have finished a book and you can do it again.