This phrase is like a mantra for organic gardeners, and with good reason. In conventional chemical agriculture, crop plants are indeed fed directly using synthetic fertilizers. When taken to extremes, this kind of chemical force-feeding can gradually impoverish the soil. And turn it from a rich entity teeming with microorganisms insects and other life forms, into an inert growing medium that exists mainly to anchor the plants roots, and that provides little or no nutrition in its own right.
Organic matter improves the fertility, the structure and the tilth of all kinds of soils.
In particular, organic matter provides a continuous source of nitrogen and other nutrients that plants need to grow. As organisms in the soil carry out the processes of decay and decomposition, they make these nutrients available to plants.