CSS/Sass variables vs. sorting by declaration (grouping selectors)?

As I go back to the basics and learn vanilla CSS, I am increasingly leaning toward favoring sorting by declaration (grouping selectors) instead of using CSS/Sass variables. In another word, I find fewer reasons to use Sass because it might get in the way of achieving the "declare only once" principle that I recently discovered. I also find the idea of not needing Sass by merely getting into a good habit of sorting/grouping rather compelling.

Am I going toward the right direction here, or am I missing something I haven't considered?