Could be just about anything, from thermostats to CPAP machines to SFP fiber transceivers. Anything that needs to be just a little bit smart (but usually not internet connected) can have one. For hobbyists, the ATMega328P was the go-to for about 15 years, before ESP32 and Pi Pico took over more recently. I'm sure Arduino helped a lot with the AVR's proliferation.

There are no one-time programmable versions of AVR as far as I know, so they can all have their internal Flash reprogrammed.

Awesome, thanks! I know them mostly from hobbyist contexts, and I noted that Atmel kind of stopped updating them after the ATMega328PU (and then failed as a company), so I wondered if they were kind of a failed product line, because there weren't enough hobbyists to sustain the product line. It's nice to hear I was wrong!