Maybe a dumb question, but what's in it for Apple for open-sourcing Darwin?

Is it purely from some license restriction of some software they use?

The next best reason is normally something like "open source code tends to proliferate", like openSSH, Linux, git, etc. But Darwin does not seem to be thriving in this way - from the page, it seems to be a small hobbiest community, and it's not like Apple needs to depend upon the community for their core OS to thrive.

I'm definitely happy Apple releases Darwin as open source, so don't interpret this as me criticizing that. I'm just trying to understand their motive.