Better Home Networking
Any time an application tries to listen on a port, and the firewall allows it and has the option set to do this, the OS should send a special control signal (such as uPnP port forwarding) to the router for it to forward that port to the listening PC. When the port is no longer being listened on the OS can tell the router to un-forward that port. If that port is already being forwarded to another PC on the LAN, then the socket listen command could return an error.
This way multiple PCs on a LAN can occasionally use the same port numbers without having to explicitly set up/change port forwarding in the router setup, or one PC can always use that port without the user having to manually set anything up.