* A professional network for practitioners, which can be useful for all of the reasons that professional networks are often useful.
* Propagation of best practices.
* Informal endorsement of schemas and standards.
* A publicly-displayable statement of common goals, best practices, or standards to which its members adhere.
* Unionization, in the sense that a group can speak with a single voice when necessary to support a particular agenda or collective goal.
* Referral network for specific geographic areas, to answer the question "we're interested in open data in government X, but are there any practitioners in our area?"
* A shared jobs board, whether for FTEs or for contractors, to help to match talent with needs of the member organizations.