If they're in the park, that means they're not in their houses. And if they're not in their houses, that means they're in a place where they can meet one another, get to know each other, and feel part of a larger community. Of course, having such a mix of people in one place has its challenges. Because we're not a country of uniform values, people bring those different values with them when they come to parks. 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.