Down and right thrust is mostly a carryover from free flight where there is no control available to correct for changes in flight condition. A free flight model had to climb under full power as long as the engine was running, then transition to a slow glide with minimum sink rate after the powered phase of flight. It further had to glide in a circle to keep it in the vicinity as it came down. The down thrust prevented too steep a climb (or even looping) when under power. Since the airplane was trimmed to circle in the glide, the side thrust made the airplane circle the opposite direction under power. Using power to circle under power in the opposite direction as the glide helped in avoiding a spiral dive onder power since there was no elevator control. A skillful freeflight modeler, by adjusting thrust and some other tweaks could reproduce a consistant flight pattern flight after flight.

With radio control, where all axis control and trim is available, side and down thrust is not actually required. BMatthews answer is right on the mark. It can be helpful in flying a model that wants to climb or turn excessively but the angles are not critical.