The Nintendo 3DS system recently received a major update and Mii Plaza has just joined the party. This update is for version 3.3 but Nintendo did not reveal any details about what was in store for users that updated.
Anyways, here are some potential reasons :
1. As a standalone application, maybe it’s part of the Nintendo Network IDs sharing update.
2. Maybe it’s for adding Miiverse functions to the Mii Plaza.
3. Use for the new update notification system function of the console.