Ducks lay eggs so that they can populate. If they didn't lay the eggs, then the species would die off because they wouldn't have anymore babies.

Female ducks lay eggs when they are somewhere between 5 and 6 months old, depending on the breed and other conditions. They can lay an egg every day, usually in the morning. You can find more information here: http://www.backyardchickens.com/forum/vi...