Worms help to increase the amount of air and water that gets into the soil. They break down organic matter, like leaves and grass into things that plants can use. When they eat, they leave behind castings that are a very valuable type of fertilizer.
Earthworms are like free farm help. They help to "turn" the soilbringing down organic matter from the top and mixing it with the soil below. Another interesting job that the worm has is that of making fertilizer. If there are 500,000 worms living in an acre of soil, they could make 50 tons of castings. That's like lining up 100,000 one pound coffee cans filled with castings. These same 500,000 worms burrowing into an acre of soil can create a drainage system equal to 2,000 feet of 6-inch pipe. Pretty amazing for just a little old worm, don't you think?
Having worms around in your garden is a real good sign that you have a healthy soil.