If you have bricked your Wii or have a severe issue that is causing things not to work correctly, you may benefit from using the NAND formatter. This really should be your LAST option after all else fails, as there are some downsides/side effects.
Although the NAND formatter is a great tool and can fix pretty much any software issue, there are some side effects.
After using the formatter, the file permissions on the NAND are not as they were originally from Nintendo. This generally isn't an issue, but if you are trying to format your Wii before sending the console to the Big N (not something I support, you voided the warranty by choice) they could possibly see this and reject your claim. Also Any Region Changer doesnt like these new permissions and will give you an error - ISFS_SETATTR -102.
If you wanted to region change a Wii that has been NAND formatted. You would need to reformat the Wii and create a setting.txt with the new region.