The default WordPress pages widget in wp-includes/widgets/class-wp-widget-pages.php works by excluding pages you don't want to include in your list. But most people have a lot of pages in their website and don't want to add the page ID to the widget each time a new page is created.
This Plugin adds a widget to your WordPress dashboard that includes a dropdown with all the pages where you can select the pages you want to include instead of excluding them by page ID's.