The idea is that they’ll stop torturing you because they don’t know of the existence of the hidden volume.
(In practice I suspect it leaves some subtle queues, but maybe perfect for a border crossing )
~"Do you have any encrypted data we can't see?"
"Yes. The entire drive is encrypted"
~"What is the key?"
"iloveapplesauce6969"
~"Well, that worked and I see your data here. What a lovely family.. is that Disney world?"
"Yes it was Timmy's 5th birthday"
~"I'm going to waterboard you"
You: "This was everything!"
Them: "We don't believe you" -rubber hose-
You: "Stop it! I planned to blow up the world trade center!"
Them: "We knew you were a liar."
To themselves: "Wow, torture works."
Rinse, repeat.