Analogue question in the linux world: Why would anyone get something in the debian package repository, when they can just release their package on their website? Because it gets added support, a bigger reach and a safer and easier installation for users?

There are special people: maintainers. They collect software from the world and package them for Debian. They often are different from original developers. Original developers might not even know that their software was repackaged. It's possible because of free software licenses. Apple can't do that even if they would want: proprietary software typically does not allow redistribution.

Good point, it wouldn't work that way with proprietary software.

Usually on the walled garden they get paid.