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.
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
Specifically page 11 the diagram at the bottom.