Question: why link nnid to nintendo account?
Answer: The main reason to link your NNID with your Nintendo Account is to merge your eShop funds. If you've got $10 or $100 left on NNID account for your Wii U or your 3DS (or both, since you can merge Wii U and 3DS), you can merge those funds with your Nintendo Account and use the money across all devices.