The overall outcome we strive for is higher quality and earlier feedback from developers resulting in a more complete, and ultimately, a better IE release for everyone. The Platform Preview is meant to provide the “right” amount of the browser to allow developers to give us that feedback. Specifically, it’s an application that only demonstrates the platform. We built it by starting with Trident and adding features rather than starting with IE and removing components. An address bar, a back button, and favorites were features we considered adding and decided were not necessary so we left them out.
These are a few diagrams showing how Trident, the Platform Preview, and the Web Browser Control relate to each other. The full Internet Explorer is yet another layer above the Web Browser Control.