Soil that is hard and packed together will not allow roots to grow well, and will cause the plants to suffer. The test tubes to the right clearly demonstrate this fact. The one on the far right was tightly compacted. The one on the far left was left looser.

You can see much deeper root growth and much healthier leaf growth in the lesser compacted soils. The grass in the heavily compacted tube is struggling simply to survive.

Foot traffic, lawn mowers, and anything else that presses down on your grass will gradually increase the soil compaction. Core aeration alleviates this problem. The openings created by the cores allow for nearby soil to loosen up and fill in the holes.