An IP gets you to a machine, but the port decides which service on it you reach. Knowing default ports like 22, 443, 5432 and 6379 makes firewall rules, security groups and ss output readable at a glance. It matters when an app works with curl localhost but not from outside, which is often a 127.0.0.1 bind or a blocked port. Also know which protocols use UDP, since a TCP-only rule can break DNS or HTTP/3.