A controversial topic – e.g., currently ~300 comments at the Andy Carvin Google+ post referenced below
Before Google+, your Google identity was really only used on Google sites. But with Google+ and the +1 button, that identity system can extend across the Web -- if you're signed in to Google+ and you +1 a particular Web site, that information can be tracked. That helps Google provide more personalized services (like search results) for you, but could also help with ad targeting.
Schmidt also said that Google believes the Internet will work better if people know that you're a real person rather than a fake person or a dog.
Andy Carvin asked the question, then paraphrased Schimdt's [sic] answer on -- of course -- a Google+ post.