1. Practicing CSS skills. While it may not serve "real world use," it's still a way to dig deep into CSS and learn a few things that could be used in the "real world."
2. In fact, Lea Verou does just that and she's a PhD CompSci researcher at MIT, as well as a creator of CSS itself and author of the 5-star book CSS Secrets. Lastly, she has dozens of Youtube videos demoing this kind of stuff. It goes deeper than just 'screwing around,' she studies pixels, color, and how they relate not just to the human eye, but to computer science as a whole.
3. It's faster and often desirable to use versus, say, images made from Photoshop. CSS often loads quicker than images on a page do, and CSS doesn't have that "boxy" and non-responsive quality that loading an image on a page does. Plus, CSS is just text and it gets rendered on the client-side, meaning it saves a ton of bandwidth versus images.
4. People are creative and artistic in general. Have you heard of chainsaw sculptures? :)
5. That all said, I hate CSS and I suck at it... But I respect those who are skilled and enjoy using it to the point that I often love looking at what they come up with.

Because it's fun, and it's a good way to practice your skills and gain new ones. They may not have a practical application, but if code is your hobby, it doesn't really matter. I love seeing the crazy CSS stuff people come up with. CSS isn't just meant for layouts, although obviously that's one of the things it does, it does so much more than that. CSS makes applications pleasant to interact with, if done correctly. You can have the smoothest running application ever, but if your UI/UX is not pleasant to interact with, people aren't going to want to interact with it.

Why do musicians practice scales, arpeggios, and rhythms when they could spend all their time performing melodies for an audience?

Its easier than svg and js.