that way you will be able to tell if the new chickens are sick, and treat them so it wont spread to the rest of your flock.
take a look at this thread: https://www.backyardchickens.com/forum/viewtopic.php?id=12751
you need to quarintine the new hens for 30 days before introducing them to you existing flock, or risk losing your flock to an unknow sickness the new hens might have