This is an interesting topic. I didn't see much difference in using React and Sass at first and would often just write plain CSS in my SCSS file. Focusing just on Sass for a moment. I started to check out their DOCS a bit and discovered that Sass has all kinds of super powers. Just one example.
$Red: rgb(130, 53, 67);
$Blue: rgb(106, 162, 181);
$Green: rgb(121, 188, 115);
$Brown: rgb(132, 102, 66);
$Purple: rgb(86, 65, 140);
background: lighten($Blue, 20%);
IDK this is just a tiny example of a color function and variables but now i don't have to manually go to a color picker and find a slightly lighter version of my blue. I find nesting useful too. I just add in new features from Sass over time. I don't have to learn it all at once.