Purpose is overrated.
Many communities with weak or no real purpose do just fine.
More important than purpose is members fulfilling basic human desires. Make more friends, have more power, earn more money, reduce the risk of pain.
Every successful community, makes it possible for members to achieve at least one of these things (often by accident). This is the element that great community strategists can map out as a blueprint and transplant from one community to the next. It's the only element that strategists can transport directly from one community to the next.
When we say it’s not about you, it’s about them, this is what we mean. Communities fulfil our basic human desires. Help your community fulfil these desires and it will thrive – regardless of the community’s core purpose.