When you create a new forest or new domain, you use the Domain Admin credentials. Through the use of the “Administrator” account you can control each and every workstation and server. You can install Exchange, System Center products and much much more. But Microsoft is probably thinking twice now about the framework they have chosen wherein the Administrator is master of your infrastructure.
As the Administrator account is so powerful, it’s a sweet spot for hackers, the target to get. And that’s probably why many people rename the administrator account to Guest (and vice versa) or something else. Many others keep the Administrator name but change the password to a very long one including special characters, but even that seems futile, by the discovery of an advanced hacking technique called Pass The Hash.