Google+ Hangouts has been the killer feature of Google's social network since day one and it still is. The ability to do group chat with full video and audio, on the web or in the mobile apps, for free is unmatched.
But, while group chat is great, sometime you don't want to have more than one source in the stream, when doing a Hangouts on Air stream for example.
Now, for these solo streams, Google is making it possible to push the video to the entire window, maximizing the used space. Previously, the participant strip was always visible, even if there was only one.
Where there's only one participant in a public Hangout, you'll only see that video. For those using Google+ to broadcast anything to a wider audience, it's a great addition.