Imagine that you've got a really big website (like mine, at 400 pages and counting). Now, imagine that at the start of every paragraph in the HTML code, you have to remember to type "<P><font face="verdana" color="black" size="3">" And, for every headline, you have to type something else. And then you type another tag entirely for each subhead. And, for each piece of sidebar information, there's a different tag. All of them have the font face, color, and size information to remember.
And, then, when you come back to your site months later to add a page or twelve, you have to remember or re-learn each of your font configurations. It becomes easy to get confused, and to start using different tags for text – soon you wind up with an inconsistent mess of text on your site.
With CSS, all you have to do is specify the characteristics for each style, and then use a simple family of tags – just <P>, <H1>, <H2>, etc. – in your HTML code. It makes it all much easier.