Appart from the incentive of money, political or social effect, and power over others, high control groups provide a pool of trusting people and a close environment over which the leader/s have a lot of control. The more ensnared someone is, the more ensnared they can eventually become.
The WT works like a totalitarian micro-society. Witnesses trust the organization blindly in EVERYTHING they do, including very personal decissions, and are kept in an infantile state for all their lives unless they decide to leave and pay the consequences.
Having this type of influence over millions of people over the world is not a force for civilization.
That is why knowing who is behind high control groups is a matter of public concern as is THEIR mental health. Tom Cruise once boasted in an interview that they are the authorities on the mind, which raises some disturbing questions when you think about what that means.