If you need an on going activity to be done on a regular basis, creating a role with one accountability may be enough (e.g. keeping up to date accounts books). Very often, at this starting point, the purpose of the role is not clearly defined.
On another way, the role may be created to reach a goal (e.g. unforgettable experience for delighted customers) with any idea about the way to do that, and a single purpose will be enough to inspire the role holder.
Actually, the initial tension drives the role proposal.