This event type falls under the Draw Event category, and although it doesn't actually draw anything, it does react to changes in the draw canvas - specifically, it is designed for reacting to the change in the Windows 8 canvas size when the game window is "snapped".
On the Windows 8 target platform it is important that you catch any resizing of the game canvas from the "snapping" of the game window (when the user drags the window to the side of the display, for example). This event will do just that and is triggered every time the canvas is changed, permitting you to add custom code here that will resize the view or reposition HUD elements as necessary. This function is provided as a convenient way to avoid having step event code that checks for these things all the time.
NOTE: You cannot draw in this event! It is triggered by resizing the canvas and is only for catching this change... any drawing that should be done still must be in one of the other draw events.