You use dynamic pages in scenarios in which it is not possible to define the layout of a page at design time. Typically you are in the process of developing a generic part of your application when desiring dynamic pages.

Example: you want to create a generic application for maintaining table records of a database. You may have a detail screen for one table record: the number of fields and their interrelation depends on the table. In order to provide a properly usable screen, you can generate the detail screen dynamically - based on meta data which comes from the database management system.