By default, whenever you create a sub-page, it will add that page below its parent page in the left-hand column navigation. By creating a new sub-section, a page will be placed at the top of the left-hand navigation and any sub-pages will appear below it. If a page has many sub-pages, then it may be a good idea to give it its own sub-section as it would otherwise clutter the left hand navigation.
To make a page into its own sub-section, navigate to the page and click on the Settings tab. Click on Appearance, choose the Menu and layout section and under Location on screen, select New sub-section.
To revert to a standard page, simply reselect the Standard Page button.