If there’s a knock on your door and when you answer, a man tells you that he’s an FBI agent and needs to search your house, would you let him in without checking his identification? Probably not.
The concept is the same when it comes to Google Authorship. Using another analogy, if you meet someone for the first time and they point at a book you’re reading and they tell you they’re the author, how do you know they’re telling you the truth? Odds are good you check their picture on the back cover. You might even ask them for their identification as proof, and compare it to the name inside of the book. Essentially, Google Authorship is an attempt to do that same kind of verification on the Internet.