> I've always been annoyed when I see a Pi with nothing connected to its GPIO header; why not just use a cheap thin client?

There have also been times when Pi's were cheap enough and x86 idled so power-inefficiently that you'd save money over a reasonable time horizon if you couldn't run your old laptops at full throttle.

Absurdly extreme example, but at one point I decided to replace a couple (maybe 3) RPi's with a single old Dell rack server off Ebay plus replaced my router with one running pfsense. I knew it would be mostly idle, that thing had 2 Xeon processors to replace 3 cheap ARM processors.

Between the 2 rack servers, my power bill went up by enough to buy a new Pi or two every month. It was like $80/month extra in power bills.