They have been bred to lay a lot of eggs. Even if you do not provide artificial light, most chickens will continue laying well beyond the normal springtime reproductive period. While some will lay well through most of the winter without artificial light, providing the latter will force more of them to do so. Some people believe this is stressful for them, but I know of no actual study on it. I do not provide artificial light because I like the idea of letting them experience the natural seasonal cycle with natural lighting, and I get enough eggs through all but a couple of weeks in mid-winter, even without artificial lighting.