Benefits of CSS

Much more flexible
Styles can be written in one place (separate style sheets) and assigned to HTML elements through class or ID properties. It's way easier and quicker to change styles across a whole site when they're defined in one place.
Lighter weight
Using CSS (like class="main-nav") creates far smaller HTML files than writing style into every HTML tags (like border="1" cellpadding="3" cellspacing="1" backgroundcolor="#ffc").