Leaves also provide bulk. Adding large amounts of them makes the soil water-retentive. In my estimation, water retention is the single most beneficial quality of any soil. It keeps plants healthy through a drought, and it keeps them from drowning in times of abundant precipitation. Foster Nature's Variety Nature favors a variety of plants.
Most pests thrive on monocultures and starve when faced with a wide selection of plants.
The principle of companion planting is based on this rule of nature. Herb gardens as well as vegetable gardens are unnatural creations. When herbs, however, are scattered throughout a vegetable garden, as they would be in the wild, their aroma protects neighboring plants from pests. The aroma of onions or chives surrounding fruit trees keep aphids and borers at a distance. No sprays are necessary for this protection, just careful planning.