Why is the amniotic egg considered an evolutionary breakthrough?

The amniotic egg is an evolutionary breakthrough because it allowed the first reptiles to colonize dry land.

Amniotes (organisms with amniotic egg) lay their eggs on land or retain the fertilized egg within the mother, while anamniotes (fishes and amphibians) lay their eggs in water. This characteristic allowed amniotes to spread around the globe. The structure of amniotic egg consists of the amnion, allantois, yolk sac, and chorion. Amnion is surrounding and protecting the embryo and it is filled with amniotic fluid; the allantois which enables gas exchange and removal of wastes; the yolk sac which provides food for the embryos; the chorion is surrounding all the other membranes and around the chorion is the albumin which prevents drying while still permitting air to reach the embryo. In the placental mammals (including humans) the allantois and yolk sac together with part of the chorion make up the placenta which connects the embryo to the uterine wall of its mother.