Someone else mentioned the clipboard sync (which is amazingly handy), but it goes so far as to the iPhone camera showing up across the macOS as both a document scanner as well as a webcam (which blows the pants off of any USB webcam on the market), and things like being able to cast your screen from your iPhone to a Mac's display.

Now I'm sure all of these things are possible on other platforms using extra software but in the ecosystem it's all zero setup, instantly integrated and quite reliable.