The most basic answer for your question is evolution. An egg serves as a protective shell to contain a developing animal embryo until it can survive on its own. Therefore, evolution has to have done something to make sure the egg stays safe and protects the baby growing inside. Animals lay their eggs in different environments depending on where they live, so egg shells have different compositions to make sure the embryo inside stays protected according to the environment it is in.Another way the egg stays safe is due to its shape.

Evolution made most animal eggs oval shaped. Many animals lay their eggs in some sort of a nest. If the egg were circle in shape, it would easily roll away, but with the unique tapered-oval shaped, the egg has less of an ability to roll away from the mother and better stays safe in its nest.