I see the advantages to having CSS variables and all, but can anyone tell me why this (Sass/Compass) would be any better than me defining variables (like grid w

I see the advantages to having CSS variables and all, but can anyone tell me why this (Sass/Compass) would be any better than me defining variables (like grid widths/colors/etc..) in something like PHP, running it through a CSS document and outputting that document as a CSS file?