Grass constantly produces organic matter through its normal growth processes and a build up of this organic matter is referred to as thatch. This thatch acts like a sponge, holding onto more water than the existing soil. As thatch builds up it creates a putting green that is soft, inconsistent to play on and is a key predisposing factor for disease. Aeration removes this thatch and the holes are filled with sand. This allows water and oxygen to penetrate the soil profile to improve root growth that as a result leads to improved plant health and better greens.
The following video shows the process and extensive labour required to fill the holes with sand after the greens have been aerated and the cores removed on 13 Hawk green: