Even though your stylesheets are cached, the first time someone visits your site a bloated CSS file can really slow your site down. This isn't too great for a first impression. As sites grow, they tend to expand with new widgets and objects that designers continue to fine-tune with more and more stylesheet options. Over time, your stylesheet can get quite bloated and be a key part of why your site downloads slower than others.
I've seen other CSS verification tools out on the web. We've used Clean CSS to reduce the file size by better organizing and minifying the data on it. When you're using a third party to analyze your site, you have to be careful, though. If they scrape one page and analyze your CSS, the tool may have you remove reduce tons of styles that are utilized on other pages.
Not the case with Unused CSS – a tool that Andrew Baldock from Mindjet, a mind mapping application, showed me yesterday. The tool crawls your site and identifies unused CSS. You can even check styles you want to keep regardless of the analysis. To top it off, you can download the stylesheet after it's run through a minify routine.