Question

Of what evolutionary advantage is it for mammals to nourish their young inside the mother prior to birth and to nurse the young after birth?

Answer 1

By nurturing the young inside the mother, they are kept in a warm, stable, environment - good for development.
Caring for their young after birth, the parent(s) ensure that their energy was not wasted in procreation (carrying the developing embryo is a HUGE energy investment).
Together these greatly increase the likelihood that each offspring is more likely to reach maturity (and reproduce), thus increasing the evolutionary fitness of the parents.