When you create a single, simple web page, you can tag each item (such as a heading or a paragraph) with whatever font, size, color, and so on that you like. It's simple HTML.
If someone then asks, "Can we use a different font for the headings?" it's a 10-minute edit.
But when you have a site with a thousand pages of corporate content and your boss arrives to tell you that head-office has determined that the "corporate style" of such-and-such font must be applied to every web page...
Then you'll have a week's worth of work ahead of you.
When they tell you that the web page must utilize a different font and color in India than the rest of the world...
You're dealing with a nightmare.
CSS takes all of the scattered formatting and layout information from the real documents and centralizes it into "style sheets."
Now, if all of your web pages are constructed correctly with CSS, you can change the font, color, size, or anything else - possibly the entire look and feel of your website - with a single-line update in the CSS file.
This concept of separating the "data" from the "presentation style" is quite powerful...
Even on a small website, there is justification in doing so.