Let's use the metaphor of a model airplane that is held together with glue. Without the glue you only have a pile of sticks - no airplane. In much the same way, your book holds together the objects in your document. Without a book, all you have are objects - no document. Your book holds the content together and creates the book structure.
Using a book template makes creating your content easy, as many of the book properties can be set in the template. Define which external publishing templates to use and include regularly used objects like a Title Page object, as well as topic objects that are common to many of your books such as a Copyright Notice or Introduction section. This way you can avoid having to add these objects to each individual book.
Use the standard templates that come with the default library, or create your own templates. Either way, it helps if you understand what the standard templates do. This topic describes the default behavior of the standard temlates.
If your library has been customized, the behavior may be different than described.