Let's take a moment to look at the interaction between the earthworm and the soil. In the ground different kinds of worms occupy different areas and plains. L. terrestris, which can exceed a foot in length, lives out it's entire life within a verticle burrow which can reach 20 feet in depth. L. terrestris comes to the surface to gather dead leaves which it pulls back down into it's burrow for food. The 'common field worm' Allolobophora caliginosa, likes the top 2 feet of the soil, and lives on a horizontal plain. Leaf or manure worms like E. foetida, can be found in only the top few inches of the soil and then usually only under rich food sources like manure piles or compost heaps. These and other surface feeders do essentially all their work above ground.

How this benefits the soil, the plants, the farmer or gardener, and so ultimately all of us, is the stuff of which books are made. I'll only touch here lightly on some of the ways all this worm activity works to improve plant growth and soil fertility, going into more depth on the subject in other parts of this site.

Humus! Worms eat dead organic materials like leaves and manure, excreting casts or castings (more on these little jewels later for sure!), which enrich the soil helping to break up clay soils, giving sandy soils water retention capabilities, freeing nutrients for plant use, and a host of other really nice stuff. The worms deposit their casts as they burrow. The casts of a healthy worm population can be measured in tons on a single acre of land over the course of a season!

While all this 'casting about' is going on, the soil is being aerated and loosened by the burrowing worms. Dead plant materials are consumed, helping plants get the oxygen they need around their roots while avoiding 'burning' caused by oxygen starvation. In the case of L. terrestris, hard pan, plow pan, and clay layers, are penetrated aiding proper drainage of the soil and bringing deep minerals to the surface.

In short, worms convert dead organic matter into plant food. As they live out their lives, they are depositing oxygenated organic humus in all the right places and in the best possible form (humus), within the soil. Worms are constantly fostering ecologically stable, sustainable, non-chemical plant growth, and improving the soil quality, making for improved plant health and vitality. Healthy plants better resist insect pests, and disease, requiring reduced use of chemical pesticides, fungacides, fertilizers etc. If you eat food, then this is important to you.

The question was about "wild" earthworms but there are earthworms which have been domesticated for use in roles ranging from fishing and stock feed, to commercial scale garbage eating and composting (vermi or vermic-composting), as we shall be seeing further on.