Once businesses go beyond the basics of setting up their Local Network and start sharing applications, files, websites, servers and other resources on to the internet; or if a firewall is introduced onto the Local Area Network, you will inherently find yourself with a need to manage "Ports". In the case of my clients we do a lot of Port Forwarding (ie. forward a connection from an external IP address to a local IP address).
Beyond the above, once you have more than one of the same service to share out (i.e. two web servers, terminal services, etc) you will inherently want to assign alternate ports. With this in mind, here is a handy link to a list of commonly used ports and more importantly "unassigned" ports: