I don't understand the uses of (CSS) preprocessors and post-processors

Question

Please I don't seem to understand preprocessors and post-processors and their applications (CSS's and that of any other language if there's any) like

Sass
Less
Stylus
PostCSS

Answer 1

From what I know about Sass, it is generally just used as an easier way of setting values to elements and organizing them within the code itself. It does so by adding variables and nested elements, which often just makes the CSS code a little easier to read and write.

As for the other ones, I would recommend checking out these sites to get a better understanding of their use in web development -
https://css-tricks.com/sass-vs-less/
https://webdesign.tutsplus.com/articles/why-i-choose-stylus-and-you-should-too--webdesign-18412
https://stackshare.io/stackups/less-vs-postcss-vs-sass