If the system we are building is not large enough to have its own CDN, we can ease a future transition by serving the static media off a separate subdomain (e.g., static.yourservice.com) using a lightweight HTTP server like Nginx, and cut-over the DNS from your servers to a CDN later.

So how does this help exactly? Isn't this just sending requests to a back-end server? Is that idea that having all of your static media on a dedicated back-end somehow faster than having both your static stuff and other stuff on the same back-end?