One could then do bad things that would be ascribed to the victim. People have lost their jobs, even their freedom, from such acts. Those with this access are perforce trusted not to abuse it this way. Yet someone has to be able to modify it. That's the crux of security.
Most computer systems feature some kind of user identification, at least as part of authorization.
Yet, with the concept of the all-powerful privileged user who can bypass everything, user identification isn't perfectly reliable. It is meaningful only insofar as you control tightly who can bypass it. My people administer their own desktops, and everyone knows the root password to the fileserver. Okay, you need to trust the telephone. But if there are only three of them, for sure, and the fact leaks from your office, the investigators will thank you for keeping the number small.