Roosters can be a pain. For one thing, even a short crowing session is likely to wake the neighborhood. Meanwhile, they are aggressive if they think something's threatening the flock, even if the perceived threat is a curious child. Many municipalities ban roosters, but it's OK. Hens get along fine without a rooster.
A rooster has two serious jobs: to protect the flock and to procreate. He protects his flock by keeping a constant eye out for danger and giving a warning call if a predator is in the area, allowing hens and chicks to run for cover. A good rooster may also lead a killer away from the rest of the flock. His second job is simply to fertilize eggs to create the next generation.
Hens lay eggs whether a rooster is around or not. Producing an egg is part of their natural reproductive cycle, just like healthy young women produce an ova, or egg, once a month. The difference is that many hens produce an egg every day. Commercial egg producers take advantage of this by keeping large amounts of hens without any roosters in order to produce infertile eggs.
Both a rooster and a hen are necessary for reproduction. In other words, any eggs a hen lays without first breeding with a rooster are not fertile. In order to produce the next generation -- to have eggs that will hatch into chicks -- hens need a rooster. For people who prefer to eat fertilized eggs, or who want eggs that will hatch, a rooster must be involved.
In the absence of a rooster to protect the flock, a hen will sometimes step into the role of guardian, keeping watch while the flock is out and about. She will sound the alarm and herd the others to safety if necessary. This lead hen will help keep order among the others and settle disputes. Some even crow.