What is the purpose of a special color?

Some brands put some specific color in their brand logo. Or if you're designing your brochure by using some specific color to give it a luxurious feel. which can not be achieved by CMYK printing. By mixing CMYK inks together in verying quantity millions of color can be made. But in Four color CMYK printing it is hard to maintain that color ton exact same, continually, to maintaining that continuity we use Special color, 

Special color :- Silver, White Gold, Olive