Google+ is not just a social media , It is powerful tool to approach your website in Search engine. The authorship tool is directly integrated with your web content as author or owner.
SEO benifits From Google+ :
Author Rank : Google+ introduce the Authorship tool to identify the author of the web content using the Contributor to link in Google+ about page. The author rank is measure according to the number of followers in your circle of Google+ page.If Author rank is high , your will get high page rank in where you have contributed your web content.
Index quickly in Google: Google+ also a social network website , so you can share your wen content link in here to get quickly index in Google Search engine .As quickly indexed in Google , as soon reader find your content in search result and Google Authorship gives your content priority over other websites.
Rich Snippet : Google+ is also responsible to view search result with rich Snippet.Google Authorship identify you as an author. This will allow your Google+ profile picture to arrear beside the search result. If you want to get rich snippet search result , you should have high google author rank and Should have Google+ account with high Author rank , I have already said that Google+ author rank is depend on how many of followers you have in google+.
Get extra Social Traffic: Today the social network website drive the huge traffic to any website . Any website can drive 30% – 60% traffic from social Network website like , Facebook, Twitter . From Google+ you can Get Extra social traffic in spite of google search result and get high your Page Rank in Search Engine.Beacuse your website traffic is a indicator of Page Rank in Search Engine.
Conclusion : webmaster can easily improve SEO from Google+ and drive Social traffic which will directly get your Page Rank high.Google verify you as author or contributor of the web content. As a result your Google profile Picture show beside the search result which increase your Brand value also.So Google+ tremendously support to improve SEO always.