Style sheets – Benefits

Speed.
Initial page may load slowly, but later pages will load fast;
because the style sheets are being transferred along with the initial page, but later pages re-use those style sheets.
Maintainability.
Maintenance time is reduced and errors are also reduced.
Change in one style sheet is reflected all around the pages.
Accessibility.
Pages that use style sheets are easier to tweak in different web browsers (IE, Netscape, FireFox, etc.)
Customization.
Web authors may offer multiple style sheets for the same web content.
Consistency.
Web authors remain consistent (unbothered) by styling or layout issues and stick themselves to tasks of content design.
Portability.
A document can easily be re-purposed or re-used for an entirely different presentation by just applying a new style sheet already available for the new environment but consistent to the web page.