Sass is very much still useful today. Mixins, used sparsely, can be very powerful and avoid large doses of redundancy. Also variables with good names make the job a lot easier than communicating around hexvalues.
Variables alone are not a reason to use Sass anymore. There are CSS Custom Properties [1] now, which are even more powerful in that you can dynamically override them in specific sub-sections of the DOM. The only downside is that Internet Explorer 11 doesn't support them (if that's important for your target audience).
[1]: https://developer.mozilla.org/en-US/docs/Web/CSS/--*