Native CSS variables are still badly supported. With SASS you also can use complicated formulas, however, native variables have some advantages:
Here are some examples what you can do just by changing CSS variable with JS - googlechrome.github.io/samples/css-custom-propert…
In future native variables will allow developers to separate CSS layout from themes. You will be able to re-use same CSS but with different variables to render something very different. Users, on the other hand, will be able to set their own layout and color themes. For now a lot of extra CSS/JS required for that.
Both native CSS variables and pre-processors are good together and definitely you will use CSS variables in the future.