Not sure if this answers your question...
Assigning a role to an Org Unit (or an HR object, such as Job Key) can have the following advantages:
1) Those users (or employees) assigned to the Org Unit will automatically be assigned to that role. This means you don't have to manually assign the role to a large group of people via SU01... it will happen after running a simple job (RHPROFL0) that evaluates the org structure.
2) By assigning a role to a user account via the Org Unit (or HR Object) means that the role cannot be removed from the User Master Record without making changes to the Users assignment to the Org Structure.
3) Using the Org structure to assign roles could mean that the assignment of roles lies with the HR department as they wuld have to maintain PD relationships.
If you only want to assign one role to one person, its easier assigning the role to the user master rather than using an Org Unit. Using the Org Structure to assign roles is beneficial if you have a large organisation and are properly using the HR PA/PD system.
