Sense of purpose is misunderstood.
You don’t set the sense of purpose for your community, you provide the platform that epitomises it.
The distinction is important. It’s the difference between “we want you to care about our new travel laptop range” and “what would be your dream travel laptop?”.
A community's sense of purposes emerges from the ground up, not the top down. Your job is to get as many people as possible involved in crafting it.