Let's run a little scenario here. Let's say that every human in the world becomes a vegitarian. So, we stop raising animals to eat. Which means that animals that have been bred to be eaten are out on their own. Look at pigs. They're not particularly skilled in hunting or gathering, because they've been raised by humans for the past few thousand years. They don't give milk likes cows or goats, so humans wouldn't continue raising them. No, they'd just go out on their own, and the whole population would die out because they can't run, they can't kill, and they're CERTAINLY not being protected by humans! So, by raising them to be eaten, we are preventing some species from becoming extinct.