Semi-trailer trucks need the elevated stacks so that the exhaust can get into the slip-stream while at high way speeds. That is where this look comes from. To my knowledge, it was popular in the '70s because of the "convoy" movies. I have seen pictures of trucks that had the same conversion from the '50s. Other than looks, all it does is put hot exhaust in the bed of your truck along with the pipe it came out of.
Remember the 80's? Those trucks [semi] puked out black smoke - the stacks were up high so you could see and breathe.
