They get installed in stuff like the Home Assistant Yellow, or the Timecard Mini.

https://www.home-assistant.io/yellow/

https://www.jeffgeerling.com/blog/2023/time-card-mini-adds-p...

I use one on a "CM4 ETHER BOARD" https://dphacks.com/cm4-ether-board-minimalist-compute-modul... which provides Ethernet, USB-C for power and micro USB for flashing an EEPROM. It doesn't have an SD card slot but it has a 2230 NVME slot on the reverse side. I program the SSD on another setup and run Home Assistant on it. Ethernet is really all the I/O needed for a small server like that.

I have another one on a Bicool Mini Base Board (A) which has a lot more I/O, albeit only one HDMI port and no USB3. It's suitable for desktop type applications with the caveat that the CM4 is barely suitable for that stuff.

Digital signage, among others. Think big flashy LED screen in store window, train station & the like. Or kiosk-style uses.

Also I've read some e-scooter sharing company used CMs for the smarts in their vehicles. Edit: might have been RPi's not CMs (but the latter wouldn't be out of place).

In short: industrial & embedded uses.

Currently it runs my home assistant installation at home, so, a bit more processing would help, but it is not a critical load that requires speed most of the time. The most annoying task is compiling ESPHome firmwares from the raspaberry, which could take several minutes.

A friend of mine builds synthesizers with them: https://www.tastychips.nl/product/gr-1-granular-synthesizer/

Build products with them. I know of a few musical instruments with CM3s, for instance.

Something which would most likely to be too niche of a market but which I would spend almost unreasonable money on would be a framework like laptop which one could upgrade by popping in a new cm every other year.