Why TCP port is required for Wide IP Pool members

Let me see if I can take a crack at this.

The BIG-IP GTM typically uses 1 or more Virtual Servers ( IP and port, even if the port is 0) whether BIG-IP or anything else as part of the GTM decision tree to decide on how to respond to a DNS based question. There are ways to use GTM to respond with a simple IP address, but in my opinion a lot of the real power that comes from GTM is from the health monitoring aspect of the product.

If you want the GTM to utilize the monitors for a virtual server in the GTMs decision making process you need an IP and service port for the service the GTM is monitoring.