I didn't hear Brees's explanation, but it seems to me like they were in two-down territory. It wasn't their last shot. Given that, they thought they might catch Dallas w/ the long ball when they maybe werenn't expecting it so much.
I think they did the same thing on the game winning TD. I was fully expecting (as I'm sure Dallas was) them to run the ball and kill some clock. I'm sure that's why that play worked so well. I think Dallas was expecting the run, too. TD Moore!