How does composting help earth?

The addition of nutrient rich organic matter is the help that compost gives to the earth. Organic content in the soil encourages the passage of air and water. So at the same time, it encourages the well being of plants and the critters of the soil food web. Soil that has adequate air and water pore space, as well as appropriate levels of nutrients, is less likely to compact or erode. It therefore is more likely to be supportive to healthy plants and soil food web critters. Healthy plants tend to lead to healthier environments, without the rampant use of chemicals; and to healthier people in the process.