The points system isn't that complicated. There's even a part of the site that explains their meaning.
- Miitomo coins are the currency you get by playing Miitomo. These coins can only be used in Miitomo.
- You earn Platinum coins by signing into Nintendo' various online services. These coins can be used for the smaller prizes such as getting 15% of a game or a wallpaper for your 3DS.
- Gold Points are earned by buying games on the e-shop. These coins can only be used to buy other games or to get a bigger discount (usually 30% or higher)
They couldn't do a one currency fits all model because it would be too hard or too easy to get certain things. I'm also sure My Nintendo was probably meant to be more for NX than it was for Wii U and 3DS and we'll probably see the program get extended even further as time goes on.
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