The "Workbook" tab can be used when you are analysing your dataset to find interesting information or when you are building tables to use in reports. In both scenarios you will find that using the scrapbook to quickly build tables, change them and remove them is very easy until you start to build up a list of tables that are interesting or ready for use in your report.
To help in this scenario you an move tables that you want to keep safe to the workbook tab by promoting them with the "Promote" button in the top menu.
Once you have build up a list of tables you wish to keep you can save them for later reuse using the "Save as" functionality to store the tables in a DDO file. These files do not store the created tables but instead they store the table request structure allowing you to reuse the tables at any time when data has been changed and they will immediately show the updated versions of the stored tables.