* A bit of secure, encrypted storage for my ssh keys, important papers, and similar stuff.

* A u2f token.

* A password manager running in a secure, controlled environment which exposes itself e.g. as a web app.

* A VPN node that exposes itself as a modem / network card.

An RPi could be all these. I maybe would prefer something like USB Armory [1] for for higher security, though it lacks a wifi interface.

[1]: https://inversepath.com/usbarmory.html