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).
Here is reason #3...
To produce next year's seeds
One of the more conservative activities associated with ecological gardening is raising "heirloom" plants. In addition to producing food, these plants also yield seeds that more faithfully replicate the parent plants. In other words, they breed "true." This is in contrast to growing hybrid plants from most commercially-grown seeds. Unfortunately, seeds saved from hybrid plants tend to produce offspring that are unpredictable regressions to ancestral forms. The gardener who saves seeds from her best heirloom plants is conserving (and even increasing) biological diversity while becoming more self-reliant.
Illustration from Gardening: An Ecological Approach. © Fred Montague