Earlier quoted context omitted.
I know we have firewalls, but in the normal desktop use-case, there are some applications that you want to be able for external clients to connect to. Skype (or other VoIP clients), Bittorrent, Game servers, etc all work better with or flat-out require external connectivity. In the V4 world, we have upnp or NAP-PMP to allow applications to open a port on the router and to have the router then forward the packets to a…
Isn't it easy enough to just have a local firewall on each machine where you open up ports for the apps you want to be public?
Or I might want some services open to my lan and only a smaller subset opened to the public (something the personal firewalls built into many OSes can't do)