There was a war on the humans' planet, Earth, between several different factions. It was bad; it killed most of humanity and the rest would die from starvation, exposure and radiation poisoning. Most of the animals and plants, too. The Oankali were passing through the Solar System, and would have let the planet die but they hoped terrestrial DNA would be able to teach them how to regrow limbs. Humans occasionally had a type of illness where some of their cells started reproducing out of control... if an Oankali ooloi could control that, it would let us regrow limbs and organs.
So they decided to risk intervening, to save what was left of Earth's plants and animals and humans, but the Oankali thought it would be cruel to let humans repopulate a planet while humans still... still had the behavioral instincts that led them to destroy themselves.
I changed everyone's mind. Not about the cruelty of the act, but that we should let humans have the choice of making something new with the Oankali, constructs like me, or in remaining as they were.