Layers are overlapping views of your scene that have objects attached to them. With layers, you can organize related elements of your scene and selectively make them visible or invisible in the 3D view. You can also template or reference all objects associated with a given layer.
For example, you can attach completed objects to a layer and template it. By turning the visibility of this layer on or off, you can see these completed objects in the 3D view only when you need to. Also, since the layer is templated, you will not accidentally select or change any of the completed objects on that layer. If you want to make changes to any objects on that layer, simply make it visible and untemplate it.