John Voorhees, MacStories:

The message from WWDC was clear: SwiftUI is the future, a unified approach to UI development designed to simplify the process of targeting multiple hardware platforms. It’s a bold, sprawling goal that will take years to refine, even if it’s eagerly adopted by developers.

And:

However, SwiftUI also raises an interesting question: what does it mean for Catalyst? If SwiftUI is the future and spans every hardware platform, why bother bringing iPad apps to the Mac with Catalyst in the first place? It’s a fair question, but the answer is readily apparent from the very different goals of the two technologies.

For non-developers, think of Catalyst (previously called Marzipan) as a development kit to make it easier for developers to port their iOS apps to macOS. But think of SwiftUI as more of a starting point to lay out apps that will run on all your Apple platforms.

John does an excellent job digging into the value of both Catalyst and SwiftUI, explaining each one’s place in the Apple universe. Both are important.

If I had to boil this all down to a single talking point, I’d say that Catalyst is the hero we need now, letting developers bring their iOS apps to the Mac, refreshing and enriching the Mac experience while we wait for SwiftUI to evolve and mature and for SwiftUI apps to become the norm on all platforms.

In a way, this model is similar to the early days of Mac OS X, when Carbon made it possible for classic Mac apps to run on OS X while we waited for developers to build and ship modern OS X apps.

A great read.