CSS preprocessors like Sass, LESS, and Stylus have significantly transformed front-end web development by enhancing the capabilities of vanilla CSS, allowing for more organized, efficient, and scalable code. Sass, being the oldest and most popular, provides extensive features and a strong community, making it a favored choice for large projects and frameworks like Bootstrap. LESS, written in JavaScript, offers easy integration and has a syntax similar to CSS, though its popularity has waned since Sass was adopted by Bootstrap. Stylus, while less popular overall, is favored among Node.js developers for its flexibility and powerful built-in functions. Despite the advancements in native CSS, these preprocessors remain widely used, with Sass leading in popularity. The choice between them often depends on a developer's project needs and personal preferences.