That's not the case for variables. The case for variables is when you have a set palette for the site, and you want the border of some elements to match the background color of other elements to match the text color of still other elements. Or you want to set padding on a set of classes, but then want to set the width of your left column to be some chosen value minus the padding of the center column. Or you want to flip your layout in RTL languages so that everything is reversed.

CSS won't let you do any of this natively.