Because the SEEPROM contains the encryption keys for the external hard drive, backing up seeprom.bin is critical for data preservation. If the Wii U hardware fails but you have a backup of your USB drive and the seeprom.bin file, it is theoretically possible to decrypt the data on a PC or move it to another console (though the latter involves complex key manipulation).
stands for Serial EEPROM (Electrically Erasable Programmable Read-Only Memory). Unlike OTP, EEPROM can be rewritten, typically up to 1 million times. It is usually an I²C or SPI-attached memory chip (e.g., 24LCxx series). otp.bin seeprom.bin