It is common these days to be able to sign into an account by signing in with your Facebook and/or Twitter account. Signing in this way allows your information to be automatically added to your account on whatever site you’re signing up on. Google is now making it so that you can do this with your Google Plus account.
The feature is not widely in use yet but you can find it on Flixter, Banjo, the Fancy, Shazam, USA Today, and several other sites. This feature will soon be added to most mobile applications (Android and IOS) because Google has now given developers the tools to do so. Google is claiming that this way of signing into accounts is a lot more secure and will reduce the amount of spam a user wil get.
Source | Google Developers