An additional course page is useful when you are providing a lot of content (descriptions, instructions, etc), and it would not fit easily within the section format on the main page. With this tool you can also divide the content within one section of the course into short chapters, each of which will be on a separate page.
Pages are easy and quick to edit. In some cases, it may make sense to make blocks of text that you want the students to read available as a page in the Pegaz system (which you can easily edit), rather than as a file (like a pdf).
However, if you have a very large amount of content, and want to include a large number of additional pages, you may consider using the Book tool, which allows any amount of pages under one title.