You may want to start and code the moment you lay your eyes on the page, but doing so will produce a coder blind to its surroundings – and that's not what I want or what the world needs.
A CSS preprocessor allows you to write CSS files in a modular way. It will be easier for you to write your code and change it in the future since you'll basically write less rules directly, through the use of concepts as variables, functions, mixins, and so on. You'll basically program your CSS, more than write it directly on the page.
It also makes maintaining complex systems easier. There are a lot of these tools available, the most famous ones being Sass and LESS.