If you want the save data synced, you need to use a Nintendo Account (and a Nintendo Switch Online subscription)
If not, it doesn't matter.
Nintendo (stupidly) chose not to store save data on the cartridge. Well okay, arguably this can be a good thing in some cases, since you had certain games on the 3DS where save data could not be deleted (Resident Evil Revelations IIRC, maybe others) so you would not be able to start a new save, or start fresh on a used copy. But apart from those very few cases, it's a hindrance. Not all games allow cloud syncing so for example for the few people that have a Switch Lite for on the go use along with a regular Switch for at home use, or have a Switch console for each TV in the house, not all of their save data can be transferred between them. If save data was still stored on the carts that would be largely a non issue. But, I digress.
NNID aren't used on the Switch, they're for WIi U/3DS.