First thing confuse me in Nuke>> Channel Set and Channel Dsiplay List

What the hell are Channel Set and Channel Display List??????
Channel Set: The channel set list lets you choose a set of colour channels to display in the viewer. By default, this is set to display the rgba set, but you can choose any channel set in the data stream.
Channel Display List: The Channel list controls which channel appears when you view the “alpha” channel. The default setting actually does display the alpha channel when you press the A key; however, you can change this by selecting any channel in the data stream.

And what is the advantage for such display setup?