Of what use is port forwarding?

It forwards data that is being received on one port to another.

Let's say somebody is sending a request to your router. You router would forward the request from the port it was received on to the specified port to the specified IP address.

Also, if you're sloppy like me, you can forward data that would normally go to a port that you can't seem to unfuck to a port that you know is listening, like making SMTP receive data from port 23 (FTP) because it's open and working.

Port forwarding is typically used for servers.

If you have multiple computers connected to a broadband router, the computers will use private IP addresses (which the internet would not know how to find). So the router makes all computers behind it appear to be from its public IP address so replies can find their way back to you.

For outgoing connections, the router assigns a port to keep track of what private IP to send a reply back to.

But if you have a server and someone wants to connect to it from the internet, your router would not know which private IP to forward it to, unless you configure the router to forward particular port(s) to the server. Or the router can be configured with a server private IP as DMZ, in which case it will forward any ports to that private IP that are not replies to outgoing traffic or not otherwise forwarded to a different IP.