Why is spreading compost better for growth in plants?

That it provides a healthy environment conducive to optimal growth is the way in which compost is related to plant growth.

Specifically, compost can be considered the end product of a decomposition process which yields dark, fresh, nutrient-rich matter for the soil environment in which plants grow. Nutrients such as carbon and nitrogen count among the necessary ingredients for healthy soils, soil food web members, and plants. Compost provides the nutrients in such a way that they can be taken in by plants. It also represents the source of both improved air and moisture passage through and retention by soil since it contributes to how soil holds together.