The Administrator is not limited by any user rights restrictions and can perform all functions.
Every organization account must have at least one Administrator user.
In terms of licensing, Project managers and Administrators are interchangeable; if there are three PM licenses, at least one must be an Administrator.
As a role, the administrator is typically responsible for:
Any change of payments for the profile (as well as any edition changes or upgrades).
Creating/editing/deleting other users.
Managing email notifications.
Organization settings under:
Organization settings are applied to templates when creating projects.
White-labeling the system (e.g apply a custom logo).