Page groups are very helpful for developers to manage application pages.
Developer can use Page Groups to club the similar kind of Pages. These groups give the a centralized management for pages,
For example if developer needs to build an application which includes around 10 to 15 modules then it will become hard to manage the pages as far as the application goes to Live. Since we have page groups, then whenever developer needs to change something in one Module then tracing the pages is really easy.
NOTE: Page Groups do not affect the application functionality but help developers to manage pages within the application.
Assigning Page Groups to Pages
select page_id, page_name , page_group from apex_application_pages where application_id = 65172 and page_group is not null;