There are multiple reasons why a passphrase can be useful.
A) Protection against theft of your backup or BitBox02
Because the passphrase is not stored anywhere on the device, it helps to protect against any attacks involving physical access to the device. Furthermore, if somebody compromised your physical copy of the recovery words (i.e. your microSD card backup or the recovery words written down on paper (optional)), they still would not be able to access your wallet unless they also know the passphrase.
B) Decoy/hidden wallets: Plausible deniability for in person attacks
As each passphrase derives a completely separate wallet you can make use of this to create decoy/hidden wallets. The most straightforward way is to store a small amount of coins in the standard wallet (no passphrase) and the rest of your coins in a passphrase protected wallet.
This gives you plausible deniability in case you are physically threatened as you can safely give up your BitBox & password. The attacker will be able to drain the standard wallet. The attacker will then hopefully leave you alone as he got what he wanted and won't suspect that you also have a hidden wallet.
C) Separation of funds into completely separate wallets
It is possible to use passphrases to create separate wallets from the same wallet backup. For example you could create a separate wallet using the "work" passphrase. However, keep in mind that such a short passphrase does not protect you in case of theft of your wallet backup as it very easy to brute force. In general we advise not to use the passphrase feature for this use case. If you require multiple separate accounts, you could use your BitBox0x2 with Electrum. An upcoming BitBoxApp release will also feature multiple accounts without the use of a passphrase.
However, we do not recommend this. Please install BitBoxApp version >4.28.1 and use the multi-account function without passphrase.