Because one day, you're going to type into Google's Search engine "what's for dinner?" And it is going to tell you.
How? Because it knows who your wife is, and it knows your wife's phone number, so it texts her saying "what's for dinner."
Example is crude, I know. But I can't think of a better way to explain what I see Google's end game is with G+.
The idea is really more that you'll go to look at Google Now when you wonder what's for dinner, and there'll be a card telling you that, and you won't even need to ask.
Though the search bit might be an intermediate step on the way there.