SwiftUI is a user interface framework that lets you design iOS app interfaces in a declarative way.
Declarative UI is best understood in comparison to imperative UI, which is what UIKit and AppKit do for iOS and macOS. The problems that come when using imperative UI frameworks are normally associated with the state of values we store in our code. In contrast, declarative programming lets us tell our application what it should do and look like in different states, and handles itself moving between those states.
Why do we adopt this technology?
Even though SwiftUI has been available since iOS 13, it has still undergone big changes with the releases of iOS 14 and iOS 15. The range of screens and apps you can build is much wider when you're no longer targeting iOS 13 users. Adoption will inevitably come when we move to iOS 14 as a minimum deployment target for features and apps.