https://www.polygon.com/2018/9/13/17856730/nintendo-switch-online-cloud-saves-explained

Cloud saves, which will allow users to store their save files online as well as on their console, are finally on their way to the Nintendo Switch, thanks to Nintendo’s new online service, the company announced during its September Nintendo Direct.

The new cloud save system will backup players’ saved games automatically. If your Nintendo Switch system breaks or stops working for some reason, your saves will still be fine and you can download them when you sign into a new Nintendo Switch.