CSS preprocessors are tools that allow us to write CSS in a more powerful and expressive way, using features such as variables, mixins, functions, and operations. They then take your preprocessor code and compile it into regular CSS that the browser can understand.

There are several reasons why you might choose to use a CSS preprocessor:

1) Code organization: Preprocessors allow you to use features such as variables and mixins, which can make your code more modular and easier to maintain.

2) Reusability: Mixins allow you to define reusable chunks of code that you can include in multiple places, reducing the amount of repetition in your stylesheets.

3) Vendor prefixes: Preprocessors can automatically add vendor prefixes to your CSS, saving you the effort of manually adding them.

4) Improved syntax: Preprocessors offer a more powerful and expressive syntax than regular CSS, which can make it easier to write complex styles.

Overall, CSS preprocessors can help you write more efficient and maintainable CSS code, but they are not necessary for all projects. It's up to you to decide whether the benefits of a preprocessor are worth the extra effort of learning a new tool and setting up a build process.