AKNative
Another reason why spot colours are used in printing is that you can get an exact colour match, because a specific colour ink is used to print with. That colour can be seen in a spot colour swatch book. CMYK colour is more a hit and miss, as the combination of the four process inks is more variable, depending on the press, the operator, and the brand of inks used.
Also the range of colours possible with spot colours is wider than with process colours (CMYK) alone. (If you are using yellow plus two spot colours, that's 3 colours by the way).