The correct answer to this open question is the following.
The Iroquois replaced people who had been lost in battle by taking captives from mourning war. The purpose of a mourning war for the Iroquois was to replace who had died.
Historians considered that the Iroquois waged war with other nations for spiritual supremacy. It was not a matter of gaining new land or supplies. That is why they had mourning wars, the kind of war against other Native tribes to avenge the Iroquois people killed in previous battles. It served the Iroquois as an act of revenge and honor the name of the people that had been killed in the war.