What is the Real Effect of Urban Agriculture?

Solnit writes that urban gardens are more than just a row of organic tomatoes:

"Thought of just as means of producing food, the achievements of urban agriculture may be modest, but as means of producing understanding, community, social transformation, and catalytic action, they may be the opposite. When they're at their best, urban farms and gardens are a way to change the world. Even if they only produced food-it's food. And even keeping the model and knowledge of agriculture alive may become crucial to our survival at some later point."

Solnit profiles several San Francisco Bay Area urban farms, including Berkeley's Edible Schoolyard Project.