As they eat the organic material, they turn it into rich nutrients by leaving castings everywhere they go. There are also compartments within the worm's body in which microorganisms multiply and are then released into the soil. They break up soil hardpans and compacted soil. Web of tunnels allows moisture to move to and through the soil, so that plants and organisms receive the water they need, but have good drainage too. Having a tunnel built for them rather than pushing through compacted soil means that roots can grow up to 7 inches a day through the tunnels.
Plants can root more deeply.
Castings provide nutrition right at the root. Converting humus to castings provides nutritional value to plants and organisms. As plants decay, they form phenols and formaldehyde which can inhibit growth in plants. Earthworms reduce these substances and generate auxins and cytokinins instead, which stimulate plant growth. Castings contain as much as 10 times the soluble, plant-available nutrients as the original soil.