How do you reset the nindtendo dsi to factory settings?

In order to do a factory reset on a Nintendo DSi, you must "Format System Memory" within the "Settings" menu. Heed the WARNING screens that are displayed before you proceed.

What's wrong with your DSi that it requires a reset? All settings should be reversible except the firmware version. I know when I had an M3i Zero I had trouble with the firmware, but it runs custom firmware anyway.