CMYK also know as the mode of printing color. As its name implies, use for printing.
It is a big difference compared to RGB. RGB is a glowing color mode, you can still see the contents of the screen in a dark room.
CMYK is a kind of reflexive color mode. How do we read the contents of newspaper? It is reflected by the sun or light to the newspaper and then reflected in our eyes. So we can see the content. It needs to have an external light source. If you are in a dark room, you can not read the newspaper.
As long as the image displayed on the screen, that is the performance of RGB. As long as the image is seen in the print, that is the performance of CMYK. Such as journals, magazines, newspaper, posters, etc . They are printed out, then it is the CMYK model.