This. Lots of useful vegetables aren't even bulk producible because of natural properties that make them hard to grow in large industrial scales. It's the micro-nutrients and supplementary food content that a community garden produces, not your major staples. You're not growing a corn field in metro Brooklyn, that's not the point. But it doesn't actually take much effort to add some locally grown lettuce to your diet from a community garden instead of shipping it all the way from California, and it reduces demand therefore lowering prices for everyone else to do so.