why they are going for more channels in image processing?

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"Channels" in image processing usually mean "colors".

You use the combination of 3 basic colors to represent a colored image: red, green, blue (RGB).

But you might need other colors, too:: eg. in printing they usually add the black (that's why it's called CYMK). Other times you might need other "colors", like infrared, ultraviolet, etc. By adding different wavelength light, you can get to hundreds of channels on the image (it's called hyperspectral imaging).

Channels might mean other things, too: like slices in a CT (computer tomography) image. There you also have more than 3 channels.

You can use channels for other things, too: define a transparency mask, or just add to the same image other results you want to keep together (eg. you can have 3 channels for the colors, 2 for the X and Y gradients, etc.)

For example, here are the spectral bands of a multispectral imaging satellite (it has 10 channels: 2 monochrome, 5 visible and 3 infrared):

Another example:

In fluorescence microscopy, you can get up to 6 different colors corresponding to different dyes used to stain your specimen. I believe 6 is the maximum with a standard (i.e. not hyperspectral) microscope.

For example here are some pictures of an irradiated human chromosome, where different regions are painted with different dyes. The order of bands indicates damage to the chromosome.

guy