If you have an editable region within your site's design that should be dynamic across your entire website such as a site footer or side bar, then you will edit that information in one place. For instance, if your site has a Site Wide banner rotator, the images in this banner rotator will automatically populate across your site.
Let's say you have a Site Wide banner rotator loaded onto the Top Content Area of your site. On every internal page on your site, the same images display. Perhaps you want a unique set of images on a particular page. You can override your Site Wide content on a page by page basis by going to one of your pages. Navigate to the Top Content Area. You will not see anything installed there because it is pulling from the Site Wide content instead. Install a banner rotator page part directly onto that page. When you add in images to that banner rotator that is installed directly on the page, it will override the Site Wide banner rotator. This works for other pages parts that might be installed on Site Wide content, like Custom Content.