It's called Streetpass, and it's a genius marketing move. When you come near another person with a 3DS and they have Streetpass enabled, the two consoles exchange a bit of information. At it's most basic level you "get" the person's Mii, or avatar in your collection. Collecting more Mii's allows you more points/abilities in some of the built in games, and that's kinda neat.
But what's even cooler? Some 3DS titles have Streetpass content. So if you have Nintendogs and walk past someone else who does (and it's based on the save file, not the actual game being in the machine, or even running) you find out about their pet. The next time you take your dog for a walk (regardless of whether that person is around anymore) you may see their dog. It can also unlock features and additional abilities in games.
In short (too late!), the more save games you have on your system, the more additional content you can get. And even if you don't have the game anymore, it'll be available if you do get the game in the future. See where I'm going? The more different games save files you have, the more you get. Not to mention the fact that anyone else you interact with gets something out of the deal down the road. My friend has Street Fighter, but I don't. If we meet, he doesn't get anything. If I have a save file for Street Fighter (and don't own the game, don't even want to) and we meet, he can get bonus stuff.
So it's in the best interest of all 3DS owners to have the biggest collection. So that's where the 3DS gamebang comes in. Plan an event, invite all your 3DS having friends, and get together for an hour or so. Everyone brings their games and shares them around. You all end up with save files, you all share your Mii's for the Mii Plaza, and everyone leaves happy.
Who's up for it?