I totally concur. This is actually our main usage on a set of 4 rpi4 and rpi5, building up ARM docker images from our products and run all testing from CI. Yes, I could use ARM-based VPS, but we wanted our own in-house CI running.
Before that, we were testing Rancher with these ones for ARM testing, too. No miniPC right now offers that, but I think they will be coming soon.
Another reason why one might want to buy one is as a build server for arm64 packages.