CMYK is for colour separations when printing and to make it simpler to mix colours to make another colour - like dark red will be made from a duotone of black K and magenta M. Dark blue can be black and cyan. Newspapers use it - if you notice on some pages have solid blocks of colours down one side of a page, those are the CMYK registration.

It is a very good way of saving money - if you push out a thousand ads using only two inks instead of three or four, you don't spend as much on printing costs. In the old days, when I started design work, if the job was CMYK - and they almost always were - the printers would ask for separations for each CMYK value, then when they print they printed each one on top of each other to make the final production. Of course you lose some colour vibrancy, but the skill is getting the mix right and as close to the original as possible.

Now I work in architecture I have to handle NCS and RAL colour conversion setups as well - which also look entirely different on screen, paper, and in real life.