With UPnP, I do not find myself doing too many port forwards but one very useful feature is this scenario. From my office, I am behind a fairly restrictive firewall that only has certain ports open. I want to be able to listen to my music server from my office via iTunes. I use Slim Devices software as a server on a Mac Mini at my house. This runs on port 9000 on the Mac. I can't access port 9000 from my office. So, I set up a port forward from public port 8000 (which is open at my office) to private port 9000 on my Mac via the router.
1) Click on NAT/QoS and then Port Forwarding:
2) Give the application a name (this can be anything so you remember it) and then assign the public port you want to use, the protocol (use Both if you do not know), the private IP address of your machine you want access to and the private port that is running on.
3) Click Enable and then Save and Apply Settings
In many cases, the private (port to) and public (port from) ports will be the same. You will only change these when you want to need to access a private service on am port that you normally can not, like my office scenario.
Port forwarding works great for remote access of computers as well via Apple Remote Desktop, Microsoft Remote Desktop, VNC, etc when you need access to a specific computer behind your firewall.
Combine port forwarding with static leases to really make your network easy to administer!