Why Dethatching and Aerating Your Lawn is a Good IdeaIt's natural to think that in order to dethatch there must be thatch somewhere. Thatch is simply the built up layer of dead grass clippings that accumulates in your lawn just above the soil. Thatch is actually good for your lawn because it helps provide a barrier to prevent disease and other harmful things that might kill the grass. Thatch also allows the soil to retain moisture by not letting water evaporate too quickly.