Yup, i know the reason to lock doors, i did in my other sim to lock cats to get into every single room and making a mess all the time... :) but what i dont understand is the purpose of giving the key to someone.
I didnt notice any difference in sim behaviour when i gave the key to a good looking lady next door. lol. I still had to invite her in.
The most logical reason to giving a key to someone is to let them pass thorugh locked doors, so you dont need to unlock them every time you want someone specific to come in. Its rather ilogical mechanic.