Skip 50 seconds into the video and you'll see the killer feature here: a seamless handoff to your companion mobile app.

It seems they're even offering analytics with it: https://developers.google.com/+/features/play-installs

Very interesting feature. App associated with web page must be free and "meet a quality threshold" determined by Google.

[0] https://developers.google.com/+/web/signin/android-app-insta...

No difference IMHO, here is the link to google+ platform API doc: https://developers.google.com/+/api/oauth

Seems like it enables sharing within your app.