What purpose did large stone heads like this one serve in the Olmec culture?

Olmec culture is known for its large stone heads. They are built in big and large basalt boulders in which each head is uniquely done and one head is not a copy to the other head. The heads are part of Olmec civilization of ancient Mesoamerica. The Olmecs are the first known civilization in America and the first to have this kind of constructions. The common reason why they made this because of their religious belief.