The main purpose of the RGB colour model is that it is used in electronic equipment such as laptops and computers. RGB and CMYK are both used to create colour, the difference between the two is that RGB is an additive colour model meaning that the colour system is created light whereas CMYK is subtractive meaning that the colour is reflected light.
RGB colour model is generally used in devices which project light and anything that has to do with web (Since CMYK is used only in printing). The picture below shows what happens when the 3 colours are mixed together: Magenta, Yellow and Cyan are created, with white final created colour, since RGB colours are additive colours.
If one where to invert the colours to, magenta, yellow and cyan one would get the RGB colours with black in the center of it, instead of white. This happens because CMYK colours absorb light.
This means that if a piece is going to be printed it should be in CMYK since it's usually the standard and it is also the best option, but if something is going to remain digital it's best to keep it in RGB since it would look better on web and LCD screens,