Gone are the days when the code we write is the code that gets published. A good example is Sass Preprocessing scripts that can add superpowers to CSS. Browsers have no idea what SCSS is or what Sass is, they only know about CSS. So, SCSS must be converted into CSS via the superpowerful Sass preprocessor. You can read more on that here https://sass-lang.com/guide under the Sass Preprocessing paragraph.
Yet to come...