Why should I use CSS? (Score 1)

You are so right, it does make sense and I can think of no good reason not to use it. CSS allows developers to:

• Separate presentation from content
• Maintain stylistic consistency throughout the site
• Make the pages more lightweight and faster loading
• Simplify future design modification
• Meet accessibility guidelines
• Follow current web standards while allowing for 'graceful degradation' in older browsers

More information, as well as why of each of the above matters can be found here.

Others have mentioned it, but I'd also highly recommend alistapart and Jeffrey Zeldman's Designing with Web Standards.