You can create arbitrary local networks with it, and isolate concerns in separate hardware.
If the "social desktop" get's compromised or even rooted, the attacker would still need to find a way through the physical router/firewall, etc. It would not be just a question of finding/using VM/container/chroot escape vulnerabilities.
You can also air gap certain endpoints. Physical security? Use FDE and pop the sd card out of the SBC and take it with you when you're out of home.
Security against rubber hose cryptoanalysis or 'contempt of the court' cryptoanalysis? Devise some way to quickly destroy the sd card when necessary. Then there's nothing to rubber hose you for.
/s? Maybe. But it's all possible today, rather easy to do, and cheap. :)