As we pressed our faces to the windows to look at the yard , I could n't help but wonder why we would need a key to unlock a door from the inside .

Question: What may be the reason a key was needed to unlock the door .
Answer: It was a security feature to prevent them from sneaking out .It was a way to remind the owner to always have their house keys .It was a mistake made by the person who installed the door .It was a way to play a prank on guests trying to leave the house .