This is to make it easier to make themes. One can start with a fairly "thin" theme and enhance it over time by adding new images and improvements.

http://www.Userstyles.org/ is the home of the "Stylish" Firefox extension. It is similar to greasemonkey in that it can change the look of a page on the fly.

Although the advantage for theme coders is that it modifies CSS so you can write your theme in the browser and and then copy paste it to a .css file in your theme instead of writing it in a text editor and saving it then refreshing the page to see your changes. Making theme development UBER fast.