There are a couple of scenarios where you might want to do this.
1) For example, say you have 10 IP Cameras on your LAN (each using Local port 80) and you want to make them all directly viewable from the internet using (default) public port 80. If you were to forward a request to public port 80 to all of the cameras, then all 10 would reply at once, the images would become jumbled, and you wouldn’t know what you were looking at 😉
So for most devices, it doesn’t make much sense to forward a single port to multiple local devices … after all, which of those device should respond to the incoming request ?
2) Some applications/protocols may add their own logic about what messages to respond to … for example where you have multiple VoIP devices; each of which is configured to respond only to its unique SIP: address. In the case of VoIP, there are a couple of work-arounds including STUN
3) And there are rare cases where an external agent determines which device should respond – such as a Failover/Load Balancing mechanism between fault-tolerant critical enterprise servers. These are advanced one-off cases.