There are as many applications as you can think up.
A lot of those tiny fanless systems end up in factory hardware as well, and they may need a powerful CPU to handle all the operations the machine has to do. Medical imaging devices need powerful processors and large memory counts. I might want something like that to run a 3D printer if it was to be cohabitated in a dusty workshop.
My mother's POS went through a few iterations. I want to say the original was a Pentium III 800Mhz or something in a standard at the time desktop chassis with the monitor sitting on top. Then a small form factor PC with a Pentium HT chip (still have that one actually, upgraded it to max memory and a new harddrive throughout its lifecycle, had to retire it since it can't run anything higher than Windows 8 (not even 8.1). And finally an AIO touchscreen PC with an i3-4130 in it.
Kind of went top of the line to top of the line CPU, than a cheap i3 could do it.
It ran the cash drawer, POS system, local database for the POS system, daily backups of the same, thermal printer, inkjet printer, credit card reader, keyboard/mouse. I believe we rigged up a signal for the cash drawer to keep working. Surprisingly expensive to replace, so it was easier to build a small transistor circuit to trigger the solenoid that could open the drawer than to pay for a new one to be installed that used a different standard for opening.