When you're building sites which use many contributed modules (+ views and panels) you can end up with a ton of CSS files being called at the same time.  Generally the main down-side to this is load-time, so most Drupal site developers choose to use the css-aggregation function (found under Site Config > Performance) that comes stock with Drupal.

The problem with using Drupal's aggregated CSS is that you end up with a crazy mashed-up chunk of CSS which can be a pain when you're still editing it live on a site in development (for example, using firefox's web development toolbar or firebug).

Asides from the load-time issue, I noticed something else whilst doing x-browser testing on a new site we're developing @ design guru; Internet Explorer 7 won't load more than 30 (or 31?) CSS files at once!  Supposedly a security feature, this annoying phenomenon makes CSS/styling dev work a pain in ie7 but fear not, there's a work-around and its as easy as installing a module!

The 'IE Unlimited CSS Loader' module will save much head-scratching - it fixes the ie7 problem and lets you still work on your site's css without using the Drupal aggregator. Nice. :)