Mystery solved!

On the question of why covered bridges are covered... from "straightdope.com".... "What you're really trying to protect in a covered bridge are the structural members--the trusses. Made of heavy timber, these are the expensive part of the bridge, and if they fall apart due to exposure to the elements, so does the bridge. An unprotected wooden bridge will last maybe ten years. Put a cover over it, however, and it'll last for centuries." Who knew!