The Constitution states that a role has one or more attributes of 'Purpose', 'Domains' and 'Accountabilities'. In what situations would a role exist without a Purpose? I do not see how a role can exist if it does not have a goal to pursue?
The minimum to create a role is a name, given the fact that a role has a purpose, whereas it is explicit or not.
As an example, H1 currently has a role without an explicit purpose (but its purpose seems pretty clear to me). https://app.glassfrog.com/roles/75447
My opinion is that in certain scenarios a tension driven subdivision of work may produce roles with pretty limited scope, the activity of which can be fully defined through a few accountabilities. In other situations a well defined purpose can work better than detailing accountabilities, and leaves more room for interpretation by the role filler. If we let go of the design upfront approach to role definition, we can easily accept that tensions will shape them in many possible ways, a role may even start its own existence with the ownership of a domain and no other specs.
I think this aspect of the Constitution is very smart, as it allows to produce an initial role configuration that's as simple as the situation requires. In the end the goal of holacracy is to set up the minimum amount of rules that are needed to run the organization in a clear and tension driven way.
I too agree with you that a roles needs somehow an implicit or explicit purpose to have meaning, but think about the existing organizational structures, do all roles have purpose there? Not in my experience. Holacracy must also give room to reflect current reality as a starting point to capture and evolve what's already happening.
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.