Why the keys?

Although I am familiar with the image and symbolism behind the phrase, "the keys to the Kingdom." I always wondered why God picked this image. However, after being left home a few times with Maria, I realized that my keys are the only thing I carry on my person when I am the only adult home. This is practical, since I would hate to be accidentally locked out, especially if Maria were locked in! My money, credit cards, and driver's license are of no avail to enter the house. Keys grant access. They separate who enters the house from who doesn't enter.