Before the designer background existed, people would tend to create a transparent layer without any other layers, and then complain when the map editor got all messy with the un-erased graphics that got smeared all over the background as a result of having no background to erase the previous frame. But you should be allowed to work on a transparent layer without having a background yet. So the "Designer Background" layer was added for the map editor to show you what parts of your map were entirely transparent and prevent the distortion/smearing that occurs when you have no background defined for your map yet. It does not get included in the compiled project, and can be deleted from your project once you have a background layer if having it there bothers you. As the name suggests, it's just a layer that's there for use in the "designer" (map editor) to serve as a temporary background in case you don't have a real one yet, or in case you want to see what your map looks like without your background displayed (hidden).
