CSS allows you to save your time: Those familiar with HTML know that that it requires from user to set the font face, size, color, style and various attributes for each of their occurances on a particular page. As a consequence, you spend most of your time typing, copying and pasting the same line of code over and over again. CSS enables you to define such details only once for any element (for instance you are able to set all h2 text on your website to be size 12, blue, Courier). The styles you have set are automatically used whenever a described element occurs.
Faster page loading time: You don't have to repeat the same code many times on your site, so there is generally less code present with CSS than with HTML only. This allows for reduced page download times.
Easier maintenance: Every time one wants to alter the style of a particular element, they can easily look it up in the style sheet and it doesn't have to be edited more than once.
Superior styles to HTML: CSS gives you access to a much wider array of attributes than just HTML.