Sorry for a dumb question, but why were these bridges covered in first place? To protect the wooden construction from rain? Never seen anything like that before ...
Back in the days before steel girders and
I-beams most longer bridges were constructed with wood. An exposed wooden bridge had a life expectancy of 10 - 15 years; a covered bridge is protected from weather and could have a life of more than 100 years. The area of the United States were I live has a whole lot of natural lumber (trees) and very little iron making wood a common construction material.