~snip~
It basically allows you to have a "hidden wallet".
Without the passphrase, there is no way to know whether there is such a "hidden wallet".
Any passphrase can be used to derive new seeds. But only the correct one will derive the seed which is being used to derive the private keys for holding the majority of funds.
Adding to what bob123 has said. @zoltanb, with the same mnemonic but different passphrases you actually may have as many "hidden wallets" as you want. Something like this:
mnemonic + passphrase
i -> Seed
i, where i= 1,2........n