Although Additive Manufacturing or more specifically 3D Printing could potentially include technologies that don’t manufacture layer by layer, in reality pretty much every single technology currently available generates at some point of the process a digitally sliced version of the 3D object to produce. Then, by any of the multiple ways that currently exist ( FDM, SLS , etc) the building material is added giving shape to the first layer, followed by the second layer directly over the first one and so on until the whole object is finished.
The very same nature of this way of creating objects makes from the layer thickness an extremely important parameter to have in mind when 3D printing your designs. If the layers are too thick, the surfaces of any given object will likely to be distorted, showing “steps” instead of a soft, smooth surface. On the contrary, if the layers are too thin, it will take a huge number of layers to build the object, translating into a longer printing time. At then end, there is a trade off between speed and quality that is necessary to adjust based on the user’s requirements.