Going back and changing the width of a menu, or font size use to be very tedious and time consuming.

However, with correctly styled site using css, many variations can be created to make a custom tailored user experience. Not only does CSS save the developer time, it saves YOU time.

CSS files can also be cached -saved to the browser- which means the user doesn't download this file every time they view a page. With a combination of this, an correct design, websites load MUCH faster and perform smoother then there purely HTML counterparts.