SpotPass was/is a free service provided by Nintendo that was only on the 3DS and Wii U consoles; Nintendo Switch doesn’t provide this. Essentially what it’s purpose was, was for Nintendo to send various pieces of content to players for their games and apps. For example, Tomodatchi Life had various time-limited costumes distributed to players on a half-monthly bases. This was some of the data I had on my console. Other examples include special distributions of special Pokemon in pokemon games. So, in a sense, you could call the SpotPass data “DLC’s”. That’s why they are important to preserve, as they are DLC content that was distributed for free. They also include notifications, since all the notifications sent to you were through SpotPass. There’s also some DLC’s for New Super Mario Bros. 2 that require you to receive a SpotPass notification in order to install.