From what I've seen over the years, parks can be powerful promoters of social harmony. Parks contribute to building community in other ways, too. When I visit other cities, I'm always struck by the amount of space, sometimes in the most distressed neighborhoods, that could be turned into parks. Where others might only see a vacant lot where a building once stood, I see what could be turned into a garden. I know it's no silver bullet, but somehow if we could go into declining neighborhoods, take back those abandoned spaces, I think we could really wake up those places and the people who live there.
Parks give people a sense of ownership and a sense of belonging.
They also give people something to care about. I've seen people band together to close streets because of the traffic disrupting their neighborhoods. But I've never seen people petition City Hall to close a park because of the traffic it brings. People want their parks, and it's the job of cities everywhere to find ways to accommodate those needs.