The device holds a private key that can be regenerated from 12 seed words.
The device itself doesn't really matter if you know these 12 words.
The key is what lets you update ownership information on the public block chain. The wallet doesn't hold any coins on the device, just a key that let's you update the chain of public ownership.
For example:
- You live in Russia and see the writing on the wall for invasion and currency collapse
- Prior to the invasion you move 90% of your wealth into a BTC wallet.
- The invasion happens and the currency collapses.
- You want to escape the country, but the border exits are guarded by corrupt guards that will steal any money/valuables you try to cross with.
- You memorize the 12 seed words of your wallet and cross the border with nothing.
- On the other side you recreate your wallet and restore access to your wealth in a safe country.
This isn't just a hypothetical, this kind of thing has already happened: https://news.ycombinator.com/item?id=29518181