Acting alone, individuals oppressed by the state, groups and institutions can't achieve much. They're easy to ignore or destroy. If they band together, they become powerful. They organize themselves. They agree on their biggest concerns and develop a plan of action.
Community organizing is a process by which individuals act in union to take power away from an oppressor.
When brands create a community, they're handing over a great deal of power. The audience is given a power over the brand they have never had before. They make demands of the brand and take actions if they're unhappy (imagine your biggest/best customers boycotting en-masse/starting a campaign against you).
So why would any brand what to do this?
Because when individuals feel they have power they also feel a sense of ownership. They become loyal and repeat buyers. They become more likely to advocate for you. They become impossible for rivals to peel them away. They give you great feedback too.
The problem is when a brand tries to build a community without understanding the shift in this power dynamic. Your community doesn't serve you, you serve the community. This means deep integration, concensus taking and opinion seeking. It means being responsive and actively resolving the issues that community highlight.
If your company isn't ready to sincerely serve a community, don't try to build one. You wont be able to take this back.