Of what use is port forwarding?

Port forwarding allows remote computers (for example, computers on the Internet) to connect to a specific computer or service within a private local-area network

You may have a private network of maybe 5 computers. One of your computers may be hosting a website on port 8080. The request to this server comes to the static IP assigned to you by your ISP but the request is also sent to a certain port on that IP (for eg: http://72.1.1.3:8080). Your router to which all your computers are connected can be programmed to forward all port 8080 traffic to the server on your internal network that is hosting the website.