One noob question.
What is purpose of private key?
Isn't enough to have only wallet.dat and to know the wallet phrase.
Private key is like the password to the coins on a specific address.
Wallet.dat is the wallet in itself and is what stores all your coins, addresses, wallet passphrase and other critically important information.
you are sure your coins are always secure for as long as you have access to your wallet.dat
At the end. Don't lose wallet.dat and don't forget wallet phrase and you are safe.