Over time the soil under your lawn becomes compacted, which impede root growth.  A healthy lawn needs air, water, and nutrients to be able to get through the thatch layer and soil to get to it root zone.
Core aeration make this possible.

Most lawns will benefit from core aeration once a season, but some heavily used lawns and soils with more clay in them can see even better results with multiple treatments. Aeration should be done when the grass is actively growing to allow new grass to fill in where the plugs were removed. If your lawn is thin ask us about including over seeding with your aeration service.