They do not.
CSS is for presentation, and presentation usually differs from site to site. That means that you either have to tweak a lot, in which case using framework does not make much sense, or you have a lot of unused code in your CSS which makes the "maintainability" point moot.<p>In my view CSS frameworks are the crutch for CSS challenged (as WYSIWYG HTML editors are for code challenged) and should be avoided.<p>I also have no idea how the word "Semantically" made its way to the title. So far from I have seen CSS frameworks add very little to the semantics, take more away, and are very likely to induce an acute case of clssitis to you HTML code. Not to mention "CSS reset" which I think was stupid idea from the very beginning.