Send files between any two devices with a screen and a camera. No wires needed, works fully offline. Some advantages:

Easy file transfers between fully offline devices

It's useful for transferring config files, secrets, pastes and short messages in airgapped environments for day-to-day use, without requiring a network connection, Wifi password or cable.

Prevents local eavesdropping and exposing a radio attack surface

QR transfers are hard to intercept - it's quite hard to eavesdrop on a sequence of QR codes without line-of-sight to the screen, in comparison to other radio protocols like Quick Share (and turning on Bluetooth also exposes your devices to unwanted Bluetooth radio attacks).

Defense-in-depth against rootkits

If the messages you're transferring are themselves encrypted (say, from an offline, airgapped phone with your security key to Facebook Messenger on an online phone), you can send encrypted messages even if your online phone or app is rooted or backdoored (all E2E encrypted apps fail here). In fact, you can use any communication channel, not just encrypted apps, and your messages will remain encrypted to anyone who doesn't have physical access to one of the security keys in the convo. Want to chat mostly in Slack (unencrypted, but beautiful UI), but keep sensitive location information fully encrypted? Easy to do with an airgapped phone connected via QR codes.

Defense-in-depth for forensic analysis

It's also useful for pulling files off compromised or untrusted computers, since the bitstream can be recorded and replayed for auditing, and the transfer protocol does not involve any USB pairing, which can compromise a driver. Payload bitstreams can be fully captured for screening or IoC (indicator of compromise) development without alerting the attacker.

There are a couple other solutions out there, but none that work offline and can be self-hosted.

Related: xkcd 3295 (Semaphore).

Source: https://github.com/chrislengerich/airgap-qr-offline.