So you're at home, and you want to connect with a friend online. Maybe it's some Minecraft and you start your own server. Maybe it's an XMPP server for a little pair programming with Saros. Whatever the case, you've likely got multiple barriers to success. Your OS firewall. Your router firewall. NAT.
You try one thing, then hope it works. Then try again. How do you know it will work? An external port tester.
That's when a website like http://www.canyouseeme.org/ comes in. It will check if your port is open from the web or not, and save you time. Saving time is good. I think in the future routers should offer this functionality themselves. Let me pretend I'm outside, and I'll tell you if I can reach you. That would be sweet.