Great questions. I like it when somebody wants to know why. It shows they are thinking. This link will probably give you more information than you ever thought you needed. It is very good and will certainly cover the gaps in my response.
Buff Hooligans Adding to your flock
https://www.backyardchickens.com/web/viewblog.php?id=2593-adding-to-your-flock
Keep it in an area where there is no contact with the other chickens or anything the other chickens use for 30 days. Don't let them share the same air space, food or water containers. Change shoes especially when going from one to the other ar at least take care of the original chickens first. You are trying to avoid transmitting anything the new chicken may be carrying and parasites, virus, or infection in general can be airborne, on your shoes or clothing, in the food or water. Watch the new chicken closely for any signs of illness and inspect regularly for parasites such as mites and lice and watch for signs of worms.
Some flocks harbor certain illnesses that the chickens in that flock have developed an immunity to, don't get sick from, or show signs of. This procedure will probably not catch many of those. It will catch most things the new bird may have recently picked up and will catch many parasites. You are not really that worried about any diseases in your flock that may be transmitted to the new bird. You are trying to prevent the new bird from bringing in anything new to your flock that can wipe it out.
You'll see posts on here where someone did not isolate a new bird and it brought disease or parasites into the flock. You'll also see posts where someone did not isolate and had no problems. What you do depends on what you consider your risks and how you consider the potential consequences. How would you react if your existing flock were wiped out? So look at the potential consequences.
The other side of the risk equation is how much risk are you really taking. A chicken from an existing closed flock that has not had any contact with any other chickens may harbor parasites that would come to light during an isolation but is not all that likely to exhibit any disease symptoms that are in its home flock that they are immune to. That is not 100% true as the stress of being relocated may trigger symptoms, but the possibility of this chicken showing symptoms are much less. On the other hand, a chicken that came from a county fair, a chicken swap, or a flock that recently had outside chickens added is more likely to have a something that would show up during the isolation.
One way to help find these "hidden" diseases, the ones that the new bird are immune to, would be to put your least valuable chicken from your existing flock in with the new bird to keep it company and hopfully pick up any new diseases.
I know this is long winded, but I do that occasionally. Hope this helps.