It used to be that being in the "Administrator" group was basically the only granularity we had to differentiate users. People complained that they only wanted to work on some things, like vandalism patrolling, or file maintenance, or creating accounts on behalf of other users.
So we unbundled, so that people could get the rights they wanted to do a particular task.
And in other cases, like with Checkuser, the Bureaucrat group, and Oversight, the advanced privileges are in fact dangerous and therefore are appropriately restricted.
Actual governance is pretty simple: the only things that are truly binding are community consensus (as determined by a sufficiently public discussion, closed at its end by one or more administrators) and decisions of the Arbitration Committee, which conducts its business on public Wikipedia pages. Administrators have some leeway in terms of blocking and closing discussions, but everything is subject to review.
NB: I'm part of the system, granted. I'm an administrator, Oversighter, and CheckUser. But these roles exist for good reasons, and I think the unbundling helps increase participation, by letting people help out without requiring full community trust in all matters.