Because sometimes they make mistakes. Lots of them. Off the top of my head, I've had them move to a different nest mid term and sit on no eggs, abandoning their own and sometimes they just quit mid term (adolescent phase). They also tend to steal eggs. My most successful technique is to break a first time broody (if under 10 months) and move all broody hens to their own clean 'apartment'.Can I ask why? I am hoping for a broody in the spring and am wondering whether it is better to let them go broody a few times first....? You know, to make sure it's not just an adolescent phase?