The main purpose of Passphrase is to create a new wallet independent of the original mnemonic and to protect it by setting a secret word known only to the user.
Passphrase is a word or sentence set by the user, unlike the initial set of mnemonic automatically generated by the hardware wallet on first use, which are randomly selected by the algorithm in the standard lexicon under the BIP39 protocol, Passphrase is set and saved by the individual user and can be any word or collection of letters, even sentences.
Passphrase can be thought of as a hardware wallet's "extra word (mnemonic + 1)" function. Even if someone knows both your hardware wallet PIN and the original pass, they can only remove your non-Pass assets from the hardware wallet if Pass is enabled, and the assets in the wallet with Pass enabled cannot be stolen.