You could use it as the logic board for a micro drone, something the size of a dime perhaps. Or other applications where weight or space are extremely limited. Another example might be a medical implant of some sort, this is small enough that it could be a part of a device that is meant to be placed inside an artery, or an eyeball, or an ear canal.

Same way you would in any other microcontroller application, but smaller, so the whole device can be smaller.

Get small enough and we can really have those bloodstream robots.

Maybe an actual useful smart ring?

I make specialty vehicle electronics. My immediate thought was very small and cheap sensors. Similar to tire pressure monitoring but wired with CAN or something similar.

In small things. Probably not very feasible for hobby projects unless you can get it soldered on when the PCB is built.

fly-sized spy drone

Wrist watch.