This is full screen mirroring, much like what chromecast got a month or so ago, or like having a miracast receiver. Basically, whatever is on your device screen you can show on the roku (or so I understand it, thats how the others work). So if you are surfing the web, you can mirror that on the roku. If you are doing a presentation, you can mirror that on the roku. If you are using an app, you can mirror that app on the roku.
Ill have to check how my miracast and chromecast do, but I am reasonably certain that they fill the screen in landscape mode. That would have more to do with the device output (I think) than the receiver.