The Why:
He was terrified of the prophecy/curse on his lineage, that the leader of the greek "gods" would always birth a legitimate (son) that would rise up and kill the father.

He did it to his dad, his dad did it, and it is basically why he's such a whore as well as why Athena birthed from Zeus' forehead, he had eaten her right from the womb as an attempt to erase her.