This feature allows you to transfer color information from one channel of the original image in another channel of the destination image. As a variant - from channel-to-channel of the same image (source and target are the same).
Let's say the challenge of the four-channel CMYK to make a single channel. You can, of course, stupid to grayscale konvertnut, but in this case, less control over color get (grayscale a little differently rendered), for example, or tipua requires that the layout was only the CMYK image and the point, and we have black and white picture podlinnosti.
Turns applyin image of C, M, Y channels, the K channel, after which the purified C, M, Y channels. Get externally the same grayscale, but along the way you can edit the channels and achieve a greater precision of the images in the target channel.
The same algorithm applies if you want out of the finished design to prepare a layout for printing in 2-3 colors (cheap mass reproduction). We break down the old color in the target channels, the junk being cleaned.
It also helps when more channels than the standard - for example, have the CMYK layout to convert to Pantones, and some idiot glued all the layers))) does Not matter, comes to the aid of channels. Create Antonova channels and throws back the color information of C, M, Y, K channels, erase last, and voila - you have a layout of the custom channels...
Well, not to mention throwing one or another of the color cast of the channel from one image to the other channel (here, applications can be mass, I have described the case from my experience).