Not only that. You can change shadow quality (adding more blur or transparency). You can control better the specular, add a better volume with AmbientOcclusion (AO) in the color channel. Theres a lot of things you can do with those layers. I saw one animation that the guy just rendered the color channel (beauty pass or Diffuse) and all shadows was added by rendering the same aniamtion only with Ambient Occlusion, in the end he multiplied AO in the Color channel to get a great depth in the objects.

O good way to start is rendering a PSD file with all channels on, dont forget to turn on SkyLight and add some Ambient Light (this light creates the AO effect), open it in Photoshop or something and watch what every layers do in the final image.

I use it a lot in my illustrations to control shadows and SSS.

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