Linking an account gives you the following benefits.
1. Allows players who were registered as under 16 years old when they start playing but have since turned 16 to automatically gain access to chat and other age-restricted features.
*Age restrictions may vary depending on region of residence.
2. Allows players to share save data between the Nintendo Switch and mobile versions of the game. *This can also be done by linking with a Nintendo Account. However, this will not give access to age-restricted features (see above).