[Xavi]

Quickly switching of themes is one reason why you would want to make it easy to change css: http://xavi.co/articles/jquery-ui-css-themes-hosted-on-cdn

I can also be quite useful during development. One cool trick to achieve something similar is to add a contentEditable attribute to a style tag: http://css-tricks.com/8618-show-and-edit-style-element/