Leaves contain high levels of carbon and small amounts of nitrogen. The microbes that decompose leaves and other types of organic matter require nitrogen for their own metabolism and growth. As long as there is an adequate amount of nitrogen and If the carbon doesn't exceeds the proper portion of protein then adding leaves can be helpful. However if carbon exceeds the proper ratio it can do more damage and cause yellow spots in the yard during the rainy season.
That is why people should also mulch their grass and leave it rather then bag it and remove it. Grass is high in nitrogen so it works well with the fall leaves that are high in carbon.