If you are in control of a web design, sometimes taking the time to edit a local file, save it, and upload it to the website via FTP can be a hassle. When you can test several different configurations in a browser before deciding which one you want to use, it will save the trouble of having to continuously upload a new file version to test it on the live site.
If you are not in control of a web design and you don't like the way a certain website looks, you can change the CSS code, provided you know a bit about the design code. Create a completely custom approach to your favorite websites!