Why do you say "Mr. and Mrs. Daniel Hastings"?

I was watching a show and when two people got married they said it like this, not only the last name that they are going to share (Mr. and Mrs. Hastings) they used the guy's first name, but the wife doesn't get his first name, right? So why do it? I don't understand...

Edit 1: WOW, THANK YOU for the overwhelming amount of responses, there are over a hundred. I am trying to read them all!