As earthworms crawl through the soil eating, they burrow and tunnel bringing improvements to the soil.

Worms:

Reduce erosion

Increase infiltration of rainwater

Improve soil aeration

Allow roots to grow deeper

Incorporate minerals and nutrients at greater soil depths

Increase microbial activity

Create a way for plants to better resist stresses such as draught and disease

Since earthworms are negatively impacted by insecticides, fungicides, herbicides, and fertilizers, landowners should be very reluctant to use any chemicals on a worm rich property.