I met with a client yesterday for a cat sit consultation and we had a discussion about the key to her home. Most of the time, clients will give me a copy of the key to their home so I have access to care for their animals. Usually, they tell me to hold onto it for future use, but sometimes they want it returned so they can use it for other guests who visit etc. My client yesterday, uses a lock box that is attached to her front door. Instead of having to give a key to each person who needs access such as cleaning lady, guest, pet sitter etc., she simply gives them the number to the lock box.
Imagine you are in an accident and are hospitalized and unable to make it home. (This would apply mostly to people who are single and live alone.) Your dogs or cats are in your home waiting to be fed, get fresh water, be walked, or perhaps need their medication. You are stuck in the hospital worried about how they are doing. With a lock box you can call a trusted friend, neighbor or your pet sitter and simply give them the number of the lock box and they can care for your "furry kids". The same would apply to a natural disaster, ie fire, etc. A neighbor could be called to rescue your animals, even though they do not have a key.
Also, if for some reason you change the lock and get a new key, you don't have to replace the ones you have lent out. It is much easier and most importantly, could save your furry kid's life.