[deleted] I don't 100% understand so I'm not sure.
I believe the reason that setting was implemented was to customize the S3 public bucket URL. All other use cases are theoretical since I've not heard of anyone using them.
Another use case I can think of would be to use a custom domain for the local storage in order to apply different settings (via Cloudflare or other proxy-based CDN for example) or just to use more domains and have more simultaneous requests.
Now technically, you could also have your local folder set up as a remote mount, and use the CDN setting to configure the public URL of that other remote file server.
Edit: Something worth noting is that (I believe) you cannot use multiple adapters at once when the CDN setting is set, since it will apply to all adapters.