What does the biology do for the soil
Various sources, including the UK Soil Association, cite "Just one teaspoon of soil can hold more organisms than there are people on the planet." These organisms play the key role to enable and to support growth of crops and other plants. That said, the biodiversity is what distinguishes the soil from dirt.
Soil is full of life – Without the life in it, it is just dirt.
The macro- and micro nutrients that plants need are generally abundant in soil particles. However, without the microbes that solubilise these minerals, the plants will not be able to utilise these.
Bacteria and fungi are the little helpers that mine the minerals from soil particles, integrate them into their biomass or release the excess back into the soil.
A large amount of these minerals is released back to the soil in their plant-available forms within the soil food web, i.e. when they are being consumed by the next trophic levels such as protozoa and nematodes.
The microbes also help to build soil structure, as bacteria can stick on the surface of various particles in the soil and form biofilms while the fungi can pull particles together and form aggregates with their hyphae. Well structured soil has improved aeration and water retention, allows plant roots to grow deeper, prevents compaction and reduce the risk of erosion and the loss of top soil.
Soil Carbon Storage
Bonus point, soil biology plays an important role in soil carbon sequestration. The atmospheric carbon is captured by the plants during photosynthesis, which allows plant growth and production of plant exudates that feed the soil microbes. Both of these mean the atmospheric carbon is transformed and stored as biomass/organic matter. While part of the carbon is released back to the atmosphere (e.g. through respiration), a healthy soil is able to store more carbon than it releases back to the atmosphere.