If you are updating from the base 3DS version to the Switch version, you gain several permanent enhancements regardless of the specific patch:
While Nintendo rarely provides exhaustive patch notes, the known changes in v1.0.3 (as of mid-2021) include: miitopia switch nsp update 103
Note for emulator users: Do not try to apply the NSP update directly inside the emulator’s file menu. Instead, use the emulator’s "Install Update to NAND" function, selecting the NSP file. If you are updating from the base 3DS