This simple process only begins to scratch the surface of what's possible using layers. The composition mode between layers is configured via a spiffy option menu in the layers dialog. The opacity over an entire layer can be easily controlled with a simple scale widget in the layers dialog that allows a number between 0-100 to be set. If it is 100, the layer is fully visible. If it is 0, only the layer behind is visible.
Layers can also have layer masks.
I don't want to try to explain these in detail, but they're already implemented and offer quite a bit of power to the skilled layer user. Layers provide a nice setting for channel information and control Layers pave the way for interesting channel manipulations. If you want to cut something from one image and paste it into another image, but only with the red and blue channels, it's no problem. If you want to view and edit only the green channel, it's no problem. If you want to view only the red and green channels, it's no problem.