Walking, playing and mowing as well as rainfall contribute to soil compaction. Roots need oxygen to grow. As the soil is trafficked, soil particles in the top few inches are compacted into a layer.
Compaction is frequently a problem with clay soil.
Less air or water can enter the soil. The result is a weaker, thinner turf and more weeds. Coring will help alleviate this condition. Use a device that removes soil cores rather than a corer that only punches holes in the soil. The holes left in the turf becomes an avenue for oxygen and water to enter.