It is used to create the crypto key, using a password based key derivation function, using the user's password fed into the PBKDF the output is the key used for

It is used to create the crypto key, using a password based key derivation function, using the user's password fed into the PBKDF the output is the key used for encryption/decryption. The users device key is mixed into that PBKDF. Without both parts of the equation, you have nothing. For your reading enjoyment: https://www.apple.com/business/docs/iOS_Security_Guide.pdf https://www.apple.com/business/docs/iOS_Security_Guide.pdf Specifically page 11 the diagram at the bottom.