I think it sends a wonderful message about enjoying food, the outdoors, and the bounty of nature, encourages people to take a more active and aware role in their diets, and carries on a tradition of agriculturalism as valued by Jefferson and others.
An additional reason why people often grow food is that it gives them a sense of self-reliance. True, the grocery store or even the farmers' market is usually more reliable than your local weather and gardening skills, and growing your own food is usually more expensive than buying it. But if you end up moving to a different place where stores aren't so reliable, if there is some kind of transportation disaster or quarantine where food can't be shipped around, if you simply want to live off the grid, or if you find yourself reconstructing civilization after a major disaster, having a vegetable garden will help provide you with food security. And it's nice to feel secure in this even if the necessity never arises.
What other reasons can you think of why growing food could be a good part of a plan or preparation?
And let's not forget the planning that goes into gardening itself. The Obamas have a very nice design for their vegetable garden. Next time I get a chance I'd like to go and take a picture of the finished product.