Because the private key is composed of 64-bit hexadecimal, which is inconvenient to record and copy. For users' convenience, mnemonic was generated. It is simplified into a mnemonic consisting of 12 words, with a space between each word. The mnemonic and private key have the same function, that is, you can import the wallet by entering the mnemonic and setting a new password. Like the private key, a wallet has one set of mnemonics and cannot be modified.