Not a member of the Apple Team, but I think one of the most obvious benefits would be being able to have your tests used across a wide range of devices, reducing how many you have to run on your local machine, especially as these can be run in parallel. Being able to build up a CI/CD pipeline to TestFlight and even to the App Store is also a brilliant feature.

Take a look at the SwiftUI Accessibility: Beyond The Basics WWDC 2021 session