The importance of a second shooter can not be stressed enough. Wedding day is mostly about capturing a moment and not staging it. Having a second shooter increases the chances of capturing a special moment (plus it is so much less stressful for the photographer having back-up). One benefit is that twice as many photographs are captured. Another is just in case something goes wrong like card failure, battery drains, someone steps in front of the camera at a crucial moment... the list goes on. Having a second shooter means the likely hood of missing something important is greatly decreased. Of all the benefits the most important for me is the crossfire. That is where the photographers line up with the subject in the middle.
In sports they put cameras on all sides. Is that necessary? Sports Illustrated sent two photographers, Neil Leifer and Herb Sharfman, to cover the Ali vs Liston fight.
Leifer took this photo.
And that is Herb Sharfman between Ali's legs taking a photo no one remembers.
But aren't Wedding's more predictable? Never is this setup more important than when capturing "first look". Why do you only want to capture one person's reaction? Here is an example from a wedding we photographed with two shooters. 4 photos from one photographer and 3 from the other. One photographer would have only been capturing half the story.
Photographer's Tip: The setup should look like a triangle from above. The two photographers should stand on one point and the subject on the other. This insures you are not shooting directly at each other.