How Port triggering is different from port forwarding, as far as I have understood, is the IP within the network to pe exposed for that port is not fixed. Router kinda searches for open port in network and when found, remembers that machine’s IP and forwards it.
It can be really use full when you dont have IP fixed for the machine within the local network. Or also in case you have same services running on multiple instances and this could work as load balancer.(May be)