The partner who didn't give birth is there to take care of two mostly helpless human beings, one who just came into to the world and one who needs rest and TLC so they can rejoin the world quickly.
We're talking about human beings. Efficiency is not part of the equation. The perinatal period is the most physically (and very often mentally as well) dangerous period in a woman's life. Women to this day die from complications in childbirth.
The first six months after birth are incredibly difficult on a woman's body and mind. Women NEED their partners' help at this time, efficiency be damned.