The most common reason for NAND backups is installing custom firmware (like Luma3DS). While modern CFW installation is very safe, any software modification carries a tiny risk of a brick. A full NAND backup ensures you can restore your console to its exact pre-mod state with one click.
Unlike a PC operating system (where you can download a generic Windows ISO), every single Nintendo 3DS console has a . The NAND is cryptographically tied to your console’s specific OTP (One-Time Programmable) memory and boot ROM. 3ds nand download