Lest we forget, Google is an advertising company. And it figures that the more it knows about you, the Internet user, the more money it can make out of hitting you with 'targeted' advertisements.
When Eric Schmidt told questioner Andy Carvin in Edinburgh that G+ was built primarily as an 'identity service', he said it relied on people using their real names. Carvin's account of the conversation uses the expression 'identity service', but whether Schmidt used it or not, the expression will be forever associated with him.
As a G+ user Google will know everything you tell it, and everything you tell your friends, colleagues, wives, ex-wives, girlfriends and boyfriends.
I won't just have the content of all your email in that Gmail account you set up under an assumed name. It won't just have that record of Internet searches, pages viewed, links clicked on and product bought associated with that machine that everyone uses.
It'll know all that and more. And it'll tie it to you personally.
It will course then tie all this in with what it gleans about you from your Android phone, your Chrome OS and any other of the myriad offerings the Internet giant will serve up.
Of course, as Schmidt point out, G+ is completely optional and no-one is forced to use it.
Just be aware of what you're getting yourself into.