Probably also to run the actual application. But a Linux kernel is a different beast than macOS versions. macOS versions are pretty stale in terms of features to change abruptly with a new release.

But I think if you keep compiling for older versions you should be able to stay on an older version for a while without newer versions of the OS refusing to run it.

It's just that sometimes new features are introduced that require you to change something in your application because there's a new or deprecated framework. Apple likes to break things to not drag a lot of legacy around.