All the stuff I used to do on a large and clunky hot power hungry noisy Intel box is now comfortably running on a cluster of Raspberry Pi's tucked away in the basement ceiling where nobody can see them. I wish they were a little faster in some cases (particularly serving the Nextcloud UI) but overall I much prefer this setup, especially since I can add Raspberry Pi's to facilitate specific roles like I would in a production data center deployment.

I've got a web server, a SAN, a couple pi-holes, and a home automation controller. I plan to spin out my databases and Prometheus onto their own Raspberry Pi's. All of them are configured using Ansible. I can recreate one in a hurry if it dies and data backed up to a ZFS partition on the SAN pi. I have a couple additional Raspberry Pi's that I temporarily hook up to the network and run a script to sync the latest ZFS snapshots over for backup purposes.

I'm sure you could find SOME use for them :)