CASSANDRA-7544 has the goal of allowing you to run multiple Cassandra instances on the same network interface (IP). Instances are disambiguated using the IP and port number.
The client wire protocol already communicated the port number in events. The system peers table has an additional column containing the port which clients need to be aware of if they want to route correctly and connect to a cluster that is configured to use different port numbers at each node.
By default the client won't using the discovered port numbers. When building the client you have to specify allowing port discovery. This nit is necessary for transitioning from a cluster where discovered port numbers are for the non-SSL port. Only the native port that supports both SSL and non-SSL traffic is propagated. In the future people will only use one port for both SSL and non-SSL, but during the transition the client needs to now if it should trust the discovered port number or use the one that is supplied in the client configuration.