The main things I use Pis for are

1. Testing images to be deployed on customer Pis.

2. Testing software on ARM64 Linux. Pis are still cheaper than used Apple Silicon Macs, and require less fiddling to run Linux. I currently have a free Oracle Cloud instance that would work just as well for this, but it could go away at any time and it's a PITA to reprovision.

3. Running Mathematica, because it's free on Pi, I only use it a few times a year, and a fully-loaded Pi 5 is cheaper than a single-year personal license to run it on any other platform.

4. Silly stuff like one Pi 3 I have set up to emulate a vintage IBM mainframe.