Earthworms like the Nightcrawler tunnel around underground and produce "earthworm castings," which means, basically, worm poop. The castings create a rich medium for plants to grow in. But earthworms do more than that. They create long burrows, which allows air and water down into the soil, and by tunneling, they move the earth around. It's like tilling the soil without exposing the dirt to erosion.
Lining the walls of earthworm tunnels are fungi and bacteria that add to the health of the soil.