I find CI very valuable even on my solo projects.

> what is CI (in practice today, not in theory)? It's just a set of commands that run on a remote machine and then the output of those commands is displayed in the browser and it also influences what other commands may or may not run. What exactly is the big deal here?

The key is hermetically/reproducibly - you don't want to run commands on some random machine, you want to run commands on a well-controlled/versioned machine that won't change under you, that you can recreate at will. Which sure you should be able to do with Nix or something, but the kind of person who doesn't want to use CI doesn't want to use Nix either.

And the other key is whose machine? Particularly when it comes to e.g. Macs, which are expensive. Maybe you have a spare server farm in your closet, but most people don't.