If you want me to get technical for you I will, but the short and sweet is with the high amount of lift from the wings if the motor were strait with the thrust line you would probably never get enough down elevator to make it flat and level, hence the down thrust, if you look down from the top you will also see there is right thrust in the plane too. If you want technical I can do a much more technical explanation but it might be too much to absorb in one sitting, lol.