Domain Name as IP Abstraction vs. Domain Name as Name of Server - /etc/host{s,name} - Why?

A domain name is an abstraction for an IP address. Regardless of which computer is at that IP, the domain name should point there. So why then do we associate a domain name with a computer using the /etc/{hosts,hostname} and/or systemd's hostnamectl? I'm definitely clueless, but this just seems wrong to me. I'm setting up my first server at home that will be publicly accessible, using a dynamic DNS for the domain name, and so came across this hosts/hostname question. I'm not the only one to [ask it](https://unix.stackexchange.com/questions/187032/why-must-a-server-know-its-own-host-name-and-domain-name) , but that person didn't get an answer.