Why is it necessary that a codling moth lays large number of eggs?

The practice has survival advantages over laying few eggs. It is similar to swarm hatching. Where so many young hatch at once that there are not enough predators to eat them all. If there were enough predators then the predators would starve when there was not a swarm available. Consequently, predator numbers are unlikely to be great enough to consume all the members of a swarm.