Basically, downthrust or sidethrust is used to reduce trim changes when the throttle is moved. If the airplane tends to go nose-up when the throttle is increased, then downthrust is added. The same if the airplane yaws when the throttle is applied. Ideally, you want the model to have a little pitch or yaw change as possible when the throttle moved.

Two full-size aircraft that had downthrust and side thrust are the Aircoupe (or Ercoupe, depending upon the maker), and the Beechcraft Bonanza. Stand 'way back, and you can see it!