Using an iPad as a Cintiq-like touchscreen for macOS is a feature that I've wanted for years as something that should "just work" out of the box as a built-in feature. I've been greatly dissatisfied with third-party solutions and their annoying drawbacks such as subscription pricing (AstroPad, Duet), hardware dongles (Luna), and a generally poor user experience (all of them.)

Sidecar is a breath of fresh air, and I hope Apple will extend it to work from Mac to Mac, much like the old Target Display mode which was unfortunately dropped from the iMac 5K. Note that iPads/iPhones have also been able to send video to macOS via QuickTime Player for years, but it is a bit clunky if you just want to use the Mac display as a mirrored iOS display.