If you've ever been around a farm in spring, you know that farmers need to plow the soil. Plowing breaks up the soil, allowing air and water to get to seeds and the roots of plants. Earthworms act like tiny plows when they live in a garden. As earthworms move through the soil of a garden, they make tunnels. Just like plowing, these tunnels allow air and water to get to the roots of plants. By adding rotting organic material like manure or compost to your garden you are providing the ideal food for worms. They will actively seek this food out and come from a far for it. Not only that, but the manure/compost will add their own nutrients and moisture too.