You've learned HTML. You've built your website. It looks just fine. Why should you bother to learn CSS? Why should you overhaul your existing site to use CSS? More to the point, why should you spend time that you could be using doing other important things learning and implementing CSS? Is it really worth it? The short answer is "probably so". Here are a few of the benefits:
* Faster downloads: In spite of dramatic increases in the numbers of people with broadband internet connections, adoption is still nowhere near universal. Research shows that if your pages take too long to download, you'll lose visitors. CSS can decrease download times in a number of ways, including reducing the total amount of markup required to format your webpages, and moving styling into a separate file which is referenced by all your webpages, and thus only needs to be loaded once.
* Less bandwidth consumption: Along with faster downloads comes less bandwidth consumption. If you're paying extra because you exceed your bandwidth quota, or are at risk of doing so, CSS can help.
* Dramatically easier site design updates: If the colors, fonts, sizes, layout, and other design elements of your site are specified in a central CSS file, you can update your entire site without having to alter every single file. The same applies to editing single files: if you want to change the size, color, font, etc. of all the <h2> headers on a page, CSS enables you to do so with a single, compact statement, rather than having to add or edit HTML tags on every single one.
* Easier creation of new webpages: When the time comes to add a page to your site, it will be much quicker and easier if you can simply reference styles from an existing stylesheet rather than having to perform all of the formatting tasks over and over.
* More control of styling: Without CSS, your webpages can only have one color for visited links, one for links being hovered over, and one for links being clicked. With CSS, you can set these colors separately for different parts of the page (for example, sections that have different background colors). CSS also enables you to set different background colors and other styling for each of these link states. These are just a few examples of the kind of styling control that CSS gives you.
I'm not saying that converting to CSS should be business priority number one. But if you'll invest a little time into learning and implementing CSS now, you'll reap the benefits for a long time.