Think of it like additive manufacturing, one tiny layer at a time.
The image below is a good summary, you can build up an object by 'printing' the layers.
This allows you to build quite complex shapes, for example items that are hollow inside or are rather intricate
The cheap consumer ones are plastic but there are high end ones that do metals. There are a few videos on Jay Lenos' garage where he has printed obscure parts for his pre-war cars.