It allows you to do math without the need of a preprocessor. With CSS Calc your imagination is the only limit to what you can do. One of my favorites is that you can use CSS Calc to properly position background images based off of the bottom right corner. Without Calc this just isn’t possible unless you add extra blank space to the image or something.
Checkout My Background!
background-position: calc(100% - 20px) calc(100% - 20px);
All I’ve got to say is I wish I knew about CSS Calc sooner!
The CSS Calc function has good browser support working in all modern browsers. Not supported in Internet Explorer 8 and below.