Why listen to port 80?
Because it's a standard. For instance, browsers work without specifying it in the address.
I don't see any danger in running an intranet webserver on port 80. Of course on machines open to the internet which provide critical services authenticated and managed through http I usually avoid port 80 but... public servers, for instance, have to run on port 80, again, because that's the standard (imagine average Joe having to type
http://google.com:62856). And when the port is another than 80, this usually gets serviced by the router/firewall with nat static routes, to a server which runs more defaults than possible (usually also port 80) on the LAN to a very high port number on the WAN. This is common network administrator practice. Having a different port on LAN doesn't add anything to security.
So, the usual main reason to change a port from default 80 is in the case that two concurrent servers are running. The logic criterion usually is: keep mainstream apps on standard defaults and provide special settings to special apps.
In our case sidu-manual is the special app, while nginx, Apache, whateverhttpd are the standard apps, candidates for port 80.
This would simplify life and do absolutely no harm to security: after all one's gotta configure his own server and there is no space for cut&paste within the realm of security.