Just read this

Security benefits and risks
Just as the PIN is used to protect your device, we can say that the passphrase serves as second-factor protection for your seed.

It only exists in your head
Because the passphrase is not stored anywhere on the device, it is impervious to any attacks involving physical access and tampering with the chip. Furthermore, if somebody compromised your physical copy of the recovery seed, they still would not be able to access your passphrase protected wallet unless they knew the passphrase.