Why does Circuits as Code exist? Couldn't this just as well have been a part of SKiDL?

Yes, maybe. But "circuits as code" is so much easier to remember. Nobody knows what "SKiDL" means.

This library and the web site also serve some other purposes. Most importantly, it contains useful subcircuits that you can include in your own designs to save yourself some effort.

It's also a teaching resource where you can read through the source code to see how SKiDL is used.

Finally, it serves as a testbed for SKiDL. If I'm creating a subcircuit and it's getting complicated, that indicates I may need to add some new feature to SKiDL. And if I run into errors errors along the way, then that helps debug SKiDL itself.

There isn't a lot in this library right now, but that will change over time. Adding a new subcircuit doesn't require a big time investment. And if you have any subcircuits of your own, please submit a pull request to add them to this library.