On a RPi I can control more aspects than I can a mini pc ITX board. I can boot straight to my program. I can write directly to frame buffers. I don’t need Linux. I don’t really need a kernel…
Here are some examples of where an RPi outshines a mini-PC (though one can still achieve the same results, just putting the box outside the box):
Coffee table Digital Touch map.
Weather Station powered by a solar panel and a LiPo battery.
ADSB receiver also powered by solar and a battery.
Arcade Cabinet that sits on a bar top with a bill reader.
Mini JukeBox at the local hacker space.
Sailing autopilot using NMEA2000 connectors.
Wearables.
Playing with high density distributed computing. (More than 5 machines)
Where the mini pc really shines is:
Storage. (NAS included)
Media PC (TV sold separately)
Gaming Console
Personal Cloud (docker + nfs + caddy + <insert personalized preferences>)
General Autopilot (sensors that need GPU support).
You have left over old PCs and don’t want to open your wallet…