Aeration helps lawns by breaking up the thatch layer. Too much thatch may house insects and diseases, prevents pesticides from reaching the pests in the soil such as grubs or crabgrass seed, and reduce the effectiveness of fertilizers and watering. Thatch is a layer of undecomposed and partially decomposed plant material tightly interwoven with living tissue between the soil surface and green vegetation. A moderate layer of thatch (less than 3/4 inch thick) is good. It protects the growing point of the grass, and moderates changes in soil temperature.
Aeration also reduces compaction problems. Compacted soil reduces the amount of oxygen to the roots, restricts root growth, and limits water absorption. Aeration creates holes in the soil allowing oxygen and water to get deep into the soil, which encourages a deeper root growth. Deeper roots improve nutrient and water intake, which encourages grass growth and results in a denser turf that helps to crowd out unwanted weeds.