DirectX is a powerful beast that enables creation of stunning graphics oriented applications (not just games). On the other hand, XAML is used to quickly create basic application UI. Having both in a single application seems like an obvious choice. For example: when building a game, you can use XAML to create level selection screens, mode selection screen, options page, etc while leaving DirectX to power the game itself. It is only natural to combine the best of the two in a single application.