They're great for tinkering with distributed systems. I.e. if you want to play with Kubernetes, you can build a master and 2 workers out of RPis for like $90. They won't have enough power to run anything substantial, but it's enough to play around with upgrading the control plane or mess with the network plane. Same thing if you want to play with failure scenarios on distributed databases, or whatever. It's doable with VMs, but every time I try to do anything fancy with VM or container networking, I end up spending hours reading documentation because my VMs refuse to communicate at all.
I also like it because they force me to use Ansible and actually make my installs repeatable. On my actual server, practically everything is some snowflake crap I did at 3am that I couldn't repeat if I had to.
I am an IT consultant and own the business. Home and work blur somewhat! I have a Dell tower (ESXi), a pfSense appliance, a micro mainboard based box, two ESP32s and a 24 port PoE+ switch in my attic. With a UPS and smoke detector. There's rather more stuff hidden away in cupboards and under the stairs. There's a 50m run of ethernet and SWA 240V (all ducted and marked etc) down to our summer house in the garden, which sports an eight port PoE+ switch, 2.4 and 5GHz wifi, and four double IP66 power outlets.
"Fleet" is a bit of an excessive term but I have four RPis at home, mainly for TV frontends for MythTV. There are rather more in the office.
The above is just a sample and not the whole story 8)
They only consume a few watts, have a footprint of a few square inches, and cost a few twenties of dollars. It's a sweet spot for doing lots of things. There are a few ways they aren't ideal, but still a great compromise.
