The new process allows iOS apps to be downloaded with only the necessary assets. If you’ve got an iPhone 6, you wouldn’t need to have the icon for the iPhone 6 Plus on your device, for instance.
Developers can also test “new iOS 9 features” in the new TestFlight.
Adding watchOS 2 to the TestFlight mix is also interesting. The new Apple Watch operating system allows for apps to be loaded onto the watch without the need for an iOS counterpart. As there’s no Apple Watch app for TestFlight, it seems the iPhone will still be the conduit for beta testing new apps there as well.
Additionally, the update lets users control notifications on a per-app basis, which is great if you’re testing multiple apps, or just don’t want to be bothered by one receiving a lot of updates.