It doesn't have much significance. Apple does this for all (major) macOS releases. macOS and iOS are built off of open source components, for which Apple releases the source code now and then. It's a courtesy for the most part - most of these are BSD licensed and therefore don't strictly require source code to be released.

These notably don't include much of mac/iOS proprietary user interface components, drivers, or system frameworks - basically everything that makes macOS/iOS remotely interesting. The source code drops are helpful for the creation of other Unix-like operating systems based on the open source macOS kernel (see also the defunct Open/PureDarwin projects), and the released apps are sometimes ported (e.g. the GnuStep project has some older version of Chess.app and others ported over). There's some off and on efforts in the FreeBSD community to port Apple's open source launchd init system [1]. All in all though, there's really not all that much interest in reusing the code from these code drops by the open source community and any efforts seem to lack staying power.

The released source most practical value is most likely derived by researchers investigating macOS/iOS internals and being able to inspect the source code for some of it. As a curiosity it looks like you're able to build and run your own XNU kernel on macOS [2] (what is this, Linux?), but I can't personally attest to the process.

[1]: e.g. https://github.com/freebsd/openlaunchd

[2]: https://kernelshaman.blogspot.com/2018/12/building-xnu-for-m...