The purpose of the QR PIN unlock method is to provide a fully air-gapped workflow for accessing your Jade wallet, without needing to connect via USB or Bluetooth. This means that your Jade device and your companion device (phone or PC) do not exchange any data over the internet or any wireless network, which reduces the risk of hacking or interception.

The scanning QR back and forth between the Blockstream website and the Jade wallet is a way of encrypting and decrypting the data that is needed to unlock your wallet. Each QR code contains a part of the data, and the other device needs to scan it to complete the data. The data is also randomized and time-sensitive, which means that it changes every time you use the QR PIN unlock method, and it expires after a certain period of time. This makes it harder for anyone to copy or reuse the QR codes. DogLikesBest

The QR PIN unlock method is more secure than just entering PIN to unlock a wallet, because it adds an extra layer of verification and protection. Even if someone knows your PIN, they still need to have access to both your Jade device and your companion device, and they need to scan the QR codes in the correct order and within the time limit. This makes it very unlikely for anyone to unlock your wallet without your permission.

You can find more information on how to use the QR PIN unlock method on the Blockstream Help Center or the Blockstream Jade website.

I hope this answer helps you understand the purpose and the security of the QR PIN unlock method.