[Trisquel-users] What's the point of a firewall?

[bm-2cwce5abrzcyn1hzwsb33z4btaxgamtogq]

Reading Linux forums I keep getting on the same remarks, usually by  
newcomers, about how this or that distribution is not perfect because it  
lacks a default installed firewall.

To me, the idea exposes them for what they are. The firewall functionality is  
not built out of any kernel binary I know. All relevant distributions come  
with some sort of repository where they can find graphical interfaces to that  
firewall functionality. And, now comes the best part: they get to block ports  
that are not enabled.

Most probably the idiot is me. So please help me understand why a desktop  
distribution does need such an app?