If you have a club Nintendo account, you can use it to get points. After that, the game is "registered" to your account. you can only register a game once, even if you have a different code from a different copy.
you can use the points to get some pretty cool stuff, actually. Here's a link to the rewards page:
Club Nintendo is awesome! You can get older style games(digital versions only for Nintendo systems, Wii, Wii U, 3DS) that haven't been remade for newer systems or actual physical items. Their stuff changes monthly, so keep an eye out.