Via a tweet from Noah Smith, a blog post noting two studies on the (positive) impact of missionaries in India (health) and Africa (literacy). Presbyterians weren't the only denomination sponsoring missionaries, but they did a lot. That's one benefit of believing one knows the truth and has the duty to spread those truths to the world. (There's downsides to such beliefs, but that's not the subject of this post.)
I think the denomination has lost that certitude; certainly it seems to be dwindling as its older adherents die.