You increase your productivity and avoid common mistakes.
If you develop several sites of the similar nature, an abstraction of CSS code can dramatically speed up your productivity, help you to avoid common mistakes and simplify the management of CSS code.

You normalize your code/class base.
You have a common “default” CSS-code and (X)HTML-markup, so you always use the same IDs and class names in your projects. Code consistency throughout a number of projects makes it’s easier for you to maintain a web-site without digging into the source code of every project you’ve ever worked on — to understand how you’ve actually built the web-site.
You have a better workflow within a team.

If a CSS-framework is well documented, it can be used within a team as a common ground-up; thus you can avoid misunderstandings and ensure an optimal workflow. The latter results in avoiding mistakes and helps you to meet deadlines without compromising on product quality and without overtime costs.

You gain an optimal browser-compatibility.
Since your CSS-framework is “bulletproof”, you don’t have to worry about browser-compatibility issues and can start coding right away.
You have a clean, well-structured and complete code.

CSS Frameworks provide you with a well thought-out foundation for your sites; using them you have the basic styling of all html-elements you’ll ever need to style from the very beginning of your projects. Have you ever forgotten about code or abbr? That’s the point.