Everyone eventually memorizes 80 and 443. Fewer people keep 5432, 6379 and 27017 in their head at all times, and mixing up a database's default port with another service's is an easy way to lose ten minutes to a connection error that has nothing to do with your code.
Defaults aren't guarantees
A "well-known" port is just a strong convention, not a rule enforced anywhere — plenty of production setups deliberately run services on non-default ports for security or multi-tenancy reasons. Knowing the default is still useful as a starting assumption when a config file doesn't specify one explicitly.
Firewall rules read differently once you know the ports
A firewall or security group rule listing "allow inbound 5432" only makes sense if you recognize that as PostgreSQL — otherwise it's just an arbitrary number in a list you're reviewing for a security audit. Having a fast reference turns rule review from "look each one up separately" into a five-minute read-through.
TeckForge's Port Number Reference is a searchable list of the ports you actually run into — web, mail, databases, remote access, common infrastructure services — searchable by number or by service name.
