Compaction is a physical process that slowly reduces the amount of oxygen contained in the soil and nutrient movements to the roots...the critical part of a healthy grass plant. Roots of the plant need oxygen, and as a product of their growth process, give off carbon dioxide. As compaction increases, less and less oxygen can enter the soil and less carbon dioxide can escape. The net result is a gradually thinning lawn until, ultimately, the soil can no longer support any turf growth.
Aeration will prevent or help a umber of problems, including compaction and thatch build-up. It opens passageways in the soil, allowing better air, water, and nutrient movement. During drought conditions, aeration helps water reach thirsty roots. When rain is heavy, it allows air to penetrate and help dry up excess moisture. Each is a stress condition for your grass.
Fall and spring are the best times to aerate...and also for over-seeding and renovating with improved varieties of cool-season lawn grasses. When the existing lawn is in fairly good condition and over-seeding is being used to thicken the lawn, one or two passes with a core aerator may be the only soil preparation required. Weak existing grass, with a greater need for seed, may require additional passes with the aerator to open the soil properly.
With more than 50% of lawns in North America more than 10 years old, most could benefit from aeration and the planting of new lawn seed varieties to produce a healthier, denser lawn.
From the Lawn & Landscape Digest