Works with your existing sign-in systems
You can prompt users to sign in to Google not only at signup, but also when they return to your site, or any other time to take full advantage of your site's social features. And, your users can connect their existing account on your service to Google even if they are already linked through another social service.
Google+ Sign-In lets you securely request access to profile information and lets users control who can see their activity.
Avoid the hassle of creating your own authentication system
Google+ Sign-In mitigates data-breach risks and reduces the burden and costs of identity security. The Google authentication window creates a trusted link between you, your users, and their Google account.
Learn more about your users and their friends
With a user's permission, Google+ Sign-In lets you improve your service by giving you access to their basic information from their public profile and the list of people in their circles.
Give users more choice
Allowing users to sign in with Google lets them select which identity they want to use in your app. By offering this choice, users are more likely to feel at ease with the decision to sign up for your service, improving conversions.