If you use an HTTP server it will automatically route whatever port you opened to 443 on its own replit.co subdomain. If you use a program with an X11 context, replit will automatically detect that and render it in its own in-page visual context; plus they allow VNC connections to that environment via NoVNC. They have an HTTP API that allows repl-instance-specific persistent storage.

I'm not a paying customer or an employee or anything but I gotta say they're pretty innovative. (I will agree with others that the documentation IS definitely their weakest point, but that in part because they have so many features.)