What was the purpose of the hens?

The hens' purpose in life is to lay eggs. Orwell uses this to show how the ideals of Animal Farm have been corrupted and to demonstrate how principled resistance to the corruption is crushed.
When Animal Farm begins, one of the grievances the hens have is the way Farmer Jones sells their eggs, to their minds murdering their unborn babies for his own profit. They want to keep control over their own eggs. However, after Napoleon takes over the farm, he decides the eggs must be sold to humans, violating the animals' principles of not interacting with humans and not using money. Napoleon also violates the concept that the hens should have control over their eggs.
The hens protest by laying their eggs on high rafters where they fall off and smash on the ground. Napoleon responds by denying the hens any food until they lay their eggs in a different place, and he decrees death to any animal who gives them even a grain of food. After five days—and the death of nine hens—the hens capitulate and do what they are told.