What it's for
Port 179 is the port BGP (Border Gateway Protocol) runs on — the routing protocol that literally holds the internet together, determining how traffic finds its way between the tens of thousands of independently operated networks (autonomous systems) that make up the global internet. ISPs, cloud providers, and large enterprises use BGP to exchange routing information about which networks are reachable through which paths.
Protocol basics
BGP runs over TCP, which is somewhat unusual for a core routing protocol — most interior routing protocols use lighter, connectionless transport, but BGP's reliability requirements (routers need consistent, ordered updates about network reachability) made TCP the right fit. BGP sessions between routers ("peers") are long-lived, persistent TCP connections over which routing updates are exchanged incrementally rather than re-sent in full each time. Misconfigured BGP announcements (accidentally claiming routes you don't actually control) have caused real, high-profile internet outages, which is part of why route filtering and validation are taken seriously among network operators.