As of now, there is no single answer as to what cogged stones were made for. Some of the earliest ideas put forth were based on the presumption that they were utilitarian objects such as net weights, oil burners, nut grinders, mace heads, or cogs for stone machines. We now believe with some certainty that they were not utilitarian.
The current view holds them to be ceremonial objects, but their meaning and symbolism are open for debate. One of the earliest ideas was that they represented sea life. From this perspective, some represent starfish, others represent fish vertebrae, while others imply jellyfish. If you visit the Bowers Museum in Orange County, you will see a convincing display of this. However, it does not explain ovoid or clover forms or the drastic range in the number of individual cogs. Another view is that cogged stones may represent notable stars in the night sky, but there is little to back this view at the moment