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

The first part of the question is the easy part: if they're kept inside the mother until they are more mature, they are more likely to survive and reproduce. There is a cost to the mother, but it is outweighed by the advantage to the offspring.

Nursing post-birth: the advantage to the young is in protection, easy access to high-quality nutrition, and again, the advantage of greater maturity (speed, size, etc) on likely survival until reproductive age.