Some are complete with cutscenes, Pelicans, the Covenant, the Flood, and the works. You can load them and then play through a Campaign-like mode. In addition, some maps are made for Custom Edition that are single player. Those are custom maps, and if you install the map too, you can play the custom map with them. You'll also notice, that when you load the list of servers, there are a lot of map names listed that you probably don't recognize. On the main menu, you'll notice that there is no option for selecting Campaign only Multiplayer. Most of the maps made for Custom Edition are multiplayer. Now players can make their own maps, or own weapons, or own vehicles.
CE's main function is to allow the use of custom maps.
