A CSS preprocessors developer uses scripting languages to extend the default capabilities of CSS for more efficient and maintainable coding. They work with CSS preprocessors like Sass, Less or Stylus to create reusable and modular code, allowing variables, mixins, functions, and mathematical operations. Their tasks include defining reusable variables, using nesting to manage code, and using mixins for repeated styles. They also help in writing future-proof code, making large CSS files more organized and maintainable, and enhancing CSS with features like loops and conditionals. Their role is crucial in streamlining and optimizing frontend development.