A hen (female chicken) does not need the help of a rooster (male chicken) in order to lay an egg. She is quite capable of doing the job on her own. See how a hen makes an egg.
A rooster is needed, however, if your goal is to produce chicks. Roosters must fertilize the eggs. You can then incubate the eggs to hatch out chicks. You typically need one rooster for every 10 hens.
Roosters also can help to protect the flock. If you have a free-range flock of chickens, roosters will sound the alert if there is a predator nearby. They are also good for keeping the hens close to home. Hens have a tendency to wander, each in her own direction.
Dr. Jacquie Jacob
University of Kentucky