You only do this if your 3DS is bricked—black screen, blue light that turns off, or a crash on boot. This process writes a NAND image back to the chip.
If you meant something else—like backing up your own 3DS NAND safely, general troubleshooting, or legal homebrew development—tell me which and I’ll provide safe, legal guidance.
You do not "download" a NAND from the internet for your specific console; you must generate one from your own system to ensure compatibility.
