Decomposition of organic matter (i.e. dead plant and animal remains) in soils is an important process in any ecosystem.
Decomposer microorganisms feed on the organic matter and break it down into its simplest components. As organic matter is decomposed, water, carbon dioxide and nutrients are released. Meaning that, any excess nutrients are released and are available for plants to use to grow.
Without this process, nutrients consumed by plants and animals would remain locked in their tissue and have no way of returning to the soil after they die; resulting in soil that is low in nutrients and of poor quality for plant growth.
It is for this reason that decomposition rate is often used in science as an indicator of soil health.