Even in the case where a centralized body is perfectly benevolent, and always acts in the citizen/user's best interest, the fact remains that mere existence of a position of power is a liability to the network. What happens if a bad actor gets access somehow? It doesn't matter how well-intentioned the admin is, if an unauthorized individual gets access to the controls.
Since the existence of a centralized power is an existential risk to any network, in the real world we see societies create rules and regulations around such institutions (such as the regulations surrounding central banks). In theory, the regulations help ensure that those in power can't just manipulate the system for their own gain: for example some big bank CEOs can't just create a few million dollars in their own bank account's without anyone noticing. We can have a separate debate on the effectiveness of this system, but at minimum this is how it is intended to work.
Bitcoin is an invention that allows for the creation of a financial network that doesn't have a centralized source of power, and thus does not need the same sort of oversight in order to be trusted. Instead the user can simply run the code, and understand that they are connected to the network, that the code's rules will be followed, and that the network's code is very difficult to change. Through this, the user can be sure that the rules that govern their value will not be manipulated, without requiring the oversight of some centralized position. Instead rules changes will only come through the nearly unanimous support of the network's users, a process which helps eliminate situations where individuals can push unwanted or unjust unjust changes on others.
If there is a centralized admin in a system of value, then users should demand some sort of auditing/oversight/regulation to ensure that they are not taken advantage of. Otherwise, the user must place great trust in the admin, and in the security of the admin's powers.