How does a Trezor passphrase prevent restoration of a wallet if an attacker knows the seed? Trezor User Manual says


  if you forget the passphrase there will be no way to recover your bitcoins. Not even the Recovery seed.


How is it that you (or an attacker who knows the seed) can't just restore a soft wallet with some other application and recover the bitcoins with the other application?
The seed is protected with a password or PIN; it precludes the possibility of stolen devices being decrypted whilst online malware/rootkits are precluded by the hardware device being separate from the compromised OS's memory pool.

From Trezor:


  TREZOR’s PIN based security system is a powerful tool to keep your
  bitcoin’s safe. If your TREZOR is stolen, the only thing between the
  thief and your bitcoins is your PIN and/or passphrase. It is
  imperative that you choose a good PIN!