The Enterprise User has all of the traits of a Professional User with added benefits. The most notable is the ability to implement Data Access Security.
You cannot implement data access security with the Security Roles Manager without an Enterprise User License.
The Enterprise User is most likely someone from the IT department that understands Role-based Access Controls and how they may pertain to your organization's business data.
They will work closely with a Business Analyst to setup the business cases to be used by the other users.