There are plenty of reasons to make backups of both the file system and database. As mentioned several times throughout the course of the book, you should back up anything that is at risk of being damaged whenever you modify code, or add a new module, or even upgrade versions of Drupal. It sounds like a real pain to do this because the vast majority of the time, nothing goes wrong with the application. However, sooner or later, for some unfathomable reason, if you don't make backups, you will get stung in precisely the most painful spot.
Most especially, corrupting or breaking a database, which in turn leads to a loss of precious data, can be a real pain in the… back end of your application! So, while it is fairly easy to back up the files on the file system by simply making copies of the directories in question, or indeed copying the entire drupal folder (whatever you have named it), it is of paramount importance that you learn how to back up the database too, because this is not...