Abortion clearly is child sacrifice. I don’t know the purpose, but my guess is as good as the next guys.
I think the purpose behind promoting it so avidly is to teach people to internalize the leftist ideal that individual humans are simply cogs in a bigger entity, that entity being "society." A human that does not contribute to society has no intrinsic right to life. A human is only allowed to live as long as they are engaged in keeping society functional. Since a baby contributes nothing towards maintaining the entity that is society, there is nothing morally wrong with killing it. And by encouraging women and girls to kill their babies on precisely the basis that those babies are useless parasites, abortion advocates are teaching them in a very tangible way that they, too, are expendable the moment they stop contributing to society.
Sometimes, as evidenced by Gloria Steinem's quote above, these leftists let on that their ultimate goal is to eliminate humans and human society completely. They don't say it so blatantly all the time--it doesn't completely mesh with their propaganda that only leftists are humanitarian and care about people--but once in a while, they let the mask slip.