Motion delights the user. Animations and other visual feedback create moments of surprise and intuition. Delights also endear the device and app to the user.
Motion adds hints towards interaction. Motion is directional: it moves forward and backward, in and out of content, leaving minimal “breadcrumb” clues as to how the user arrived at the present view. For example, the Panoramic Entrance animation can draw the user’s eye across the main navigation as the app opens, giving the user a clue about how to operate the Panorama control.
Motion gives the impression of enhanced performance. When network speeds lag or the system pauses to work, animations can make the user’s wait feel shorter.
Motion adds personality. The well-considered Windows Phone UI uses motion to create the impression that an app is concerned with the here and now, and helps counteract the sensation that the user is burrowing into nested hierarchies.
Motion adds consistency. Transitions can help users learn how to operate new applications by drawing analogies to tasks that the user is already familiar with.
Motion adds elegance. Animations can be used to let the user know that the phone is processing, not frozen, and it can passively surface new information that the user may be interested in.