In fact, the mnemonic and private keys are functionally identical.
The nature of the mnemonic is also a string of random numbers (128-256 bits), but because the random number form and the private key form are not convenient to use, the Bitcoin community has adopted the BIP39 protocol to allow random numbers to be converted into words by specific coding.The words in the library.
(Bitcoin improvement proposals, referred to as BIP, is a design guidance document that provides specifications for the Bitcoin community to improve bitcoin and its operational processes and external environment characteristics)
Mnemonics do not need to be case-sensitive, but are more common words. They are much more convenient to use than private keys. So in recent years, mnemonic wallets have gradually become mainstream wallets.
Another advantage of the mnemonic wallet is that a set of mnemonics can be derived from N private keys, each of which can correspond to a currency. If you have 30 currencies (BTC, ETH, LTC, EOS, etc.), you don't need to record the private key separately for each currency. Just record a set of mnemonics to control all assets.
PS: The ET wallet will display both the mnemonic and the private key to the user. It also provides Chinese and English mnemonics (Chinese version is still under development), but in general it is good to back up the mnemonic.