Things I've used mine (plural) for at various points in time, in no particular order: Wireguard & ssh entrypoint into my home network. Pi hole. Orchestrating turning on/off IP-based lightbulbs or plugs (since I'm too cheap to get a gateway for ZigBee/Threads/$IOT_protocol) - automatically turning on my Christmas lights at sundown daily after querying an API for my local Civil twilight time was good fun. "NAS server" connected to a cheap USB 5-disk JBOD device. Hosting a low-intensity crawler that ran into blanket IP-range black-listing issues when hosted on cloud-providers. Hosting a Gitea/Forgejo server: I no longer star projects on Github - I mirror them locally and keep them synced, storage is really cheap. Periodical syncing my backups from NAS to cloud. As a digital "tape recorder" for broadcast radio using FM receiver, aux cable, and USB sound card. Twitter Spaces recording for time-shifting conversations I wanted to listen to later; I beat Twitter's record feature to the punch!. Twitter crawler/archive bot for a niche community.

Basically anything that I wanted to automate and was not demanding on compute would get assigned to a Pi.