Super Smash Bros. for Nintendo 3DS has StreetPass functionality because of course it does! It's a 3DS game! Whenever you pass by someone who owns Smash, they'll become an opponent in a special mode called StreetSmash. Each opponent, up to 12 at a time, is represented by a little disc, and you'll battle them in a top-down arena. You're still trying to knock your opponents off the edge, but now the combat is akin to bumper cars.
IGN has a short overview of the mode in the video above. For your efforts, you'll earn coins which can be used to purchase trophies. Even if you play in practice mode, you'll still earn coins, albeit not as much. You might even earn a custom move or two for your fighters in the main game!
Super Smash Bros. for Nintendo 3DS comes out October 3. The Wii U version may arrive on November 21.