Oh, potentially if you wanted to give the 2nd key to someone for safekeeping, they couldn't do anything with it because they wouldn't be able to generate the wallet. But as you say, passphrase achieves this also
Discussion
You are totally correct; but
Think of this way, you can lose your Seed Phrase & some one with knowledge can potentially sign in especially if bip39 if I’m correct. With a hardware/software as a second signature you save your from a bad situation. Sorry I make sense, I am driving in between lights so I’m trying to write fast lll