Why use SASS when CSS supports variables.

CSS variables are still very new and if you care about supporting old browsers (you should) they are unusable.

https://caniuse.com/#feat=css-variables

I've recently started integrating a combination of SASS and BEM naming conventions into projects at work (as opposed to plain CSS), and although there was a small learning curve, I've enjoyed and benefited from what I've learned so far. The things I found immediately useful were nesting and partials/imports, but I know I've still only scratched the surface.

Edit: Also, the projects I've used it on so far are smaller-scale websites, but I plan to scale use up to larger projects.