When leaves start to build up too much, to the point where no light is getting through, they start to harm your lawn. Source: residentialwastesystems.com.
While perennials, shrubs, trees, bulbs, etc. tolerate and even prefer it when their roots are covered with dead tree leaves in the fall, lawn grass is not as accepting. You have to remember that turf is an artificial environment maintained only by considerable human labor. Since grass continues to photosynthesize right through the fall until the ground freezes, it’s therefore important to regularly sweep up fall leaves and to keep doing so until the lawn does stop growing.
Of course, a scattering of dead leaves is not going to be particularly harmful to a lawn, but when leaves build up to the point where no light is getting through, that also means the lawn is no longer its daily dose of solar energy. And a buildup of dead leaves also inhibits air circulation and that can lead to lawn diseases. When there are so many leaves you can’t see the lawn, you should remove the leaves without too much delay.
If the layer of leaves is relatively thin and patchy, though, rather than remove the leaves, why no use them to “feed” the lawn? Simply run over the lawn with a mulching mower … without a bag. It will reduce the leaves to tiny pieces that will soon work their way to the ground, in between the blades of grass, thus enriching the soil. Talk about turning a disadvantage into an advantage!