One benefit is it could prolong the amount of time you get eggs. As they age they slow down, but a friend of mine has 9 year old hens that lay here and there.
As for adding new chickens. It's best to quarantine first. Let them see each other but no touching. Then after a few weeks add the new ones at night. The new group already know each other so they will stick together, but your original chicken needs time to adjust.
As for adding new chickens. It's best to quarantine first. Let them see each other but no touching. Then after a few weeks add the new ones at night. The new group already know each other so they will stick together, but your original chicken needs time to adjust.