Maps were added in Alpha 6, The Map Update. They can guide your way and show you the explored areas, but most importantly, give the 3D sense to the game, a feature which is planned to be put in the game in more than one way. You can also see the nearby terrain in the background, which is going to be an accurate representation of the area further back. To switch to a different "level" you will have to find a path and press up or down, according to its direction. However, some paths will be blocked and you will have to unblock them with different ways (ex.: explosive crate). The map icon is at the bottom left of your screen, with the other inventory icons. When double click the Map UI will appear and show you a map.
Reading a map is very usefull. It shows you the terrain that surrounds you, the direction you are facing, the places you have been to, places you can go to explore and dead-ends.