What was the need for animals to evolve from oviparity to viviparity?

Answer 1

When an oviparous animal lays eggs their choice is to

stay with the nest at all times and go without eating, leave the nest to eat leaving the eggs unprotected, have a mate that shares the responsibilities of

protecting and nurturing the eggs, doing nothing and leaving...etc. Even with all these possible protections the results can be lost eggs or an injured or dead adult. The less care that is taken, the more eggs must be laid in an attempt to

have a few survive to adulthood.

Think of the advantage of carrying offspring on the move...by protecting itself the parent protects it's offspring...a much easier task.

More complex animals, that have fewer offspring that require more nurturing and training after birth are at an advantage by being viviparous as more development can take place before birth.