Using books inside books (sub books) makes it easier to manage large documents. Common chapters or sections such as a glossary, or introduction section, can be reused easily in other books across your documentation set. Any change to the structure of the sub book such as adding, removing, promoting or demoting topics, is automatically reflected in any other books that the sub book is used in.
This approach is also recommended when there is more than one author working together on the same project. Only one person can obtain a lock on a single object at a time. While that person has the object open, others can only open the object as read-only so cannot make any changes to it. In the case of a Book object, this means that only one person can make changes to it's content at any one time, such as adding or removing topics. Other users can open topics within the book and modify these provided they are not open (and thereby locked) by someone else, but they cannot make changes to the book's structure.
Sub books can also be used to achieve a modular documentation approach. For example, let's say you have two different software products, Product A and Product B. You release three different configurations of your software products: