You get a domain name (from a non-technical standpoint) for marketing and usability reasons. You could if you like forgo the domain name and just use the IP address of your server, but no one would remember it and many would be skeptical to visit it.
The domain registrars allow you to convert your address into something brandable, I think of it this way do your offline marketing materials say this is from "123 made up street, example town USA" or do they say this is from "Examples R Us at 123 made up street, example town USA"