Sensors. Especially sensors in your living space where fans or other noise from the proper server would be distracting, or in a tight space - inside your HVAC, for example - where a proper server wouldn’t fit.

Media front-end. Most of those SBCs are more than enough to run a kodi or jellyfin frontend, fanless for minimum distraction.

Robot. Low power requirement so it could be mobile; but there are lots of stationary possibilities. GPIO libraries are great for running servos and there’s tons of libraries to facilitate.