While revealing a new partnership with SD card maker SanDisk, Nintendo has also revealed that, moving forward, some Nintendo Switch games will require a MicroSD card in order to experience the full content of certain upcoming games.
In an announcement, Nintendo noted that "a microSD card will be needed for certain Nintendo Switch games that contain an especially large amount of content and require additional storage for players to enjoy the full experience." At least one upcoming release, NBA 2K18, will be such a game requiring a MicroSD card in the system, as evidenced by the game's updated box art.
http://uk.ign.com/articles/2017/09/01/some-nintendo-switch-games-will-require-memory-cards