Without this update, your game remains at (base cartridge or dump). Many custom ROM hacks (like Pokémon Ultra Sun & Moon - Photonic Sun/Prismatic Moon ) require the v1.2 update as a base before patching.
: Some users find that if the game crashes with the update, running it once without "Game Patching" enabled in the Luma3DS menu and then re-enabling it can resolve the conflict. Why the 1.2 Update Matters pokemon ultra moon update 12 cia work