Finnbennach vrs Donn Cuailnge

Ochall is the king of the fairy people the "Connacht" and Bodb is the king of the Munster sid. they are both friends until people begin to argue about which one is the most powerful. They have magical powers and transform themselves in to maggots; then they go to the water and are dranked by cows; lastly becoming Finnbennach and Donn Cuailnge the bulls of The Tain. The war of the Tain is motivated by the existence of these two bulls,in which at the end Donn Cuailnge ends up killing Finnbennach and then dying him self. My question is: what is the purpose of these two bulls in the story.? Had queen Mebd's greed been instigated by the fact that these two bulls sought to continue their fight on other grounds then their own or is it pure coincidence that a battle is fought over them?