Each individual player on Nintendo Switch can create their own user account (up to 8 per system), to manage personal settings and save data.
Linking a Nintendo Account to a user account also provides access to the following online features*:
Access online game modes (online multiplayer, global rankings, DLC, etc)
Use any applications that require Internet access (including Nintendo eShop)
Use the Nintendo Switch Parental Controls application to manage parental controls through your smart device
Exchange friend codes with other players
Post screenshots to social media services
Earn My Nintendo points for purchases made on Nintendo Switch
* Some online services will also require a paid subscription.