A page from the Google+ help center explains the reason of the new Google navigation bar:
The Google+ bar, which appears at the top of Google products, is your connection to Google+. You can share what's on your mind, view your Google+ notifications, access your profile, or jump to a range of other Google products. For case, to get to Google+, all you have to do is click +[your first name].
When you're signed in and look at the Google+ bar, you'll see your full name or email address displayed with a photo or avatar next to it. This helps you recognize which account you're now signed in to. You can sign in to numerous accounts at once and switch between them using the Google+ bar.
One of the most attractive feature of the bar is notifications:
When you receive a notification, the notification area in the Google+ bar will turn red and show the figure of new notifications. If you click the notification area in the Google+ bar, you’ll see a synopsis of your recent notifications. When you click a notification, a preview of the event that generated the notification will emerge in the drop-down menu. You can take action on each notification right from the notifications menu, like commenting on a post or adding up someone to a circle.
The navigation bar has been morphed into the Google+ bar and it ought to be more useful. Maybe at some point the bar will take in notifications for Gmail, Calendar, Google Docs and other Google services.