From the desk of important beginner concepts:
You have a website with 100 pages on it. All 100 pages use the same style.css file. You'd like to change the background color of every single page. You make one adjustment in the CSS file, and that background color adjustment will be reflected across all 100 pages. You don't need to edit each of those pages individually. That's the core benefit behind CSS: abstracting the design away from the markup.
Now you want to make another change to those 100 pages. You'd like to include the publication date underneath the title of each of the pages. That is something you'll need to edit the HTML to do. If those 100 pages are based on a template, as they would be when using a CMS (Content Management System), you can make one adjustment to the template file and the date adjustment will be reflected across all 100 pages. That's the core benefit behind a CMS: abstracting the content away from the markup.
The point is that once a website is any more than one page, there are going to be shared resources and it's time to use a CMS. Just as the zen garden taught us that using CSS is vital to allow design freedom and make redesigns easier, the ultimate freedom comes from also using a CMS where we also aren't locked to any specific HTML. HTML isn't for content these days, it's for describing content. Databases are for content.
I have made this scientific chart to drive home the awesomeness of this all.