Accessing a website via the webservers public IP from a PC within the same LAN

Accessing a website via the webservers public IP from a PC within the same LAN

If two machines reside on the same LAN (a PC and a Webserver) why is it necessary to enable ‘NAT loopback’ on the router in order for the PC to access the website hosted on the webserver using the routers public IP (which port forwards on port 80 to the webserver)? This article on Dynamic DNS illustrates the setup perfectly www.dyndns***/support/kb/loopback_connections.html