At times while the fallen leaves are often a site to see, they also present hazards for your yard if left to sit. Worse yet, leaves can accumulate and get wet, leading to mold growth and attracting pests. This also causes your lawn to suffocate under that layer of leaves, to prevent your lawn from being smothered its best to have them removed or mulched. Not to mention grass definitely needs sunlight along with any wet leaves will cause mold.