In blender the often called layers are objects and layers are scenes. The Outliner display what we often call of layers. They have different names because they do not work the same way.
In Gimp or other image editor programs layers are made on top of which other and are always visible in Blender only active layer is visible. In Linux they are called different environments, containing several loaded programs in which the current is active.
Though they can be option in use, one may choose to use them or not. Because the Outliner display a form of layer similar to Image Editor. Since some artist may opt to use the Layer environment, is also necessary to understand them, but they often are not used.
The series is mainly on how to tweak models, but Is necessary to understand how they work in order to be able to edit or tweak 3D models that work on this type Layer environment.
How to Render Layers : Youtube.com