Google+ (pronounced and sometimes written as Google Plus /ˈɡuːɡəl plʌs/) is a social networking and identity service that is owned and operated by Google Inc.
WHY DO YOU WANT TO BE ON GOOGLE +?
If you are unable to be on Google Maps/Places/Business because you DO NOT want to establish your home address as a public registered business (and many wouldn't), then this is for you! Like Facebook, you establish a Page. After your page is set-up you can share it on your site.
Link to your website
Linking your Google+ Page and your website is useful because It provides Google with information that we can use to help determine the relevancy of your site to a user query on Google Search.
Your site will become eligible for Google+ Direct Connect.
Note: If you have a local Google+ Page, you should use the local verification process to add your business information to Google Maps and display the verification badge on your Google+ Page. You may also choose to link your website to the page using the process outlined below.
Google has described Google+ as a "social layer" that enhances many of its online properties, and that it is not simply a social networking website, but also an authorship tool that associates web-content directly with its owner/author. It is the second-largest social networking site in the world after Facebook. 540 million monthly active users are part of the Identity service side, by interacting socially with Google+'s enhanced properties, like Gmail, +1 button, and YouTube comments. In October 2013, Google counted 540 million active users who used at least one Google+ service, of which 300 million users are active in "the stream".
In a 2013 survey, 30% of surveyed smartphone users used the Google Plus app at least once a month. 92% of US smartphone users had visited a Google web site or app in August 2013, according to another survey.
UPDATE 2016: Until Google kills Google+ it is still a valuable place to register your existence as a company. Unlike Facebook, this listing is open to search, and therefore better for SEO.