People do all manner of wacky stuff with Pis that could be more easily done with traditional machines. Kubernetes clusters and emulation boxes are the more common use cases; the former can be done with VMs on a desktop and the latter is easily accomplished via a used SFF machine off of eBay. I've also heard multiple anecdotes of people building Pi clusters to run agentic development workflows in parallel.

I think in all cases it's the sheer novelty of doing something with a different ISA and form factor. Having built and racked my share of servers I see no reason to build a miniature datacenter in my home but, hey, to each their own.

I concur with this. The novelty of the Pi is getting a computer somewhere that you normally wouldn't due to the size and complexity. GPIO is a very nice addition, but it looks like conventional USB to GPIO is a thing so it's not really a huge driver to use a Pi.