I have three chicks that were raised by their mother. She ran off and hid under our shed and hatched her eggs. Since we saw her every day, I didn't notice she wasn't sleeping in the barn at night (she tended to hide in the rafters anyhow)
All three chicks are doing great! We didn't see them until they followed their mama to the food trough one morning. I was so pleasantly surprised. They are now at leasy 6 months old, still all sleep together in the barn, hang out together, but mama kicked them out when they were about 3-4 weeks old and started making them fend for themselves. They learned to perch much sooner than broodered chicks, and are much better foragers than my hatchery chicks.
That being said, this was one of the handful of birds that sleeps in my barn, and lays eggs wherever she chooses. Most of my egg layers lay in a coop built for that. If I chose to leave a broody hen on a nest in there, several things might happen a) when they did leave the nest to get food and water, another bird might come along and peck the eggs, break the eggs or otherwise do mean things b) IF she was successful and hatched them, they would be stuck in the nest, our nest boxes are high off the ground, and no little chick could get out. That means, mama would have to leave them behind to go and get food. They would have gotten killed by other hens.
Have you ever put younger birds in with adult birds? The bigger birds always pick on new additions to the flock. We had some hens get into our brooding pen one day, and they pecked several of the baby chicks to death. This is why most people put chicks in the brooder, to protect them from the rest of the flock when mama can't protect them.
We got lucky that Friend bird successfully hatched 3 chicks. She had a nest on the ground under our shed, and it's a miracle she wasn't eaten by a skunk or coyote. As soon as we saw her with them, we moved them all back into the barn, where she protected them. There were other hens around, but we provided food and water close to her, so that she wouldn't have to leave her babies unattended where someone else could hurt them. It was wonderful to watch, but I am not sure it will ever happen for us again.