The vertical heat exchanger gives you great efficiency and makes maintenance and operation easy. The point of the heat exchanger is to get the heat from the firebox into the water jacket. As the exhaust passes through, that heat is transferred through the steel to the water jacket. Small particles may be contained in the exhaust, in a vertical heat exchanger they drop to the bottom and they are very easy to clean out through the reaction chamber.
Watch Mark describe the differences between a vertical and horizontal heat exchanger in this short video.