In Gardening: An Ecological Approach I list several important reasons to grow a garden. My post on April 12, 2013 briefly explained reason #1 (to grow food for health) and my post on May 1, 2013 outlined reason #2 (to enhance soil fertility). The post on May 10, 2013 gave reason #3 (to produce next year's seeds).
Here is reason #4...
To Provide for Other Organisms
Many other organisms live in the ecological garden besides edible plants. These include microbes, earthworms, soil algae, spiders, fungi, insects, toads, and birds-- to name only a few. To encourage beneficial insects, for instance, the gardener may plant a continuously blooming array of flowers to provide refuge and alternative food sources for pollinators and predators. A variety of insects in balanced populations is desirable. A weed patch is useful, productive, and beautiful in its own way. In the garden, diversity promotes health and stability.
Illustration from Gardening: An Ecological Approach. © Fred Montague