It's for when you Streetpass people. If you carry your 3DS out with you, it might communicate with another 3DS nearby and let you view the other person's home. So if you go to the store, and someone an aisle or two away has their 3DS in their pocket or bag, then they might appear in your Happy Home Showcase next time you play.
It doesn't exchange houses over the Internet, so you won't see people just by adding their Friend Codes.
PenguinGeek is the coolest penguin ever (pun intended rofl)