Upthrust or downthrust are used to reduce or eliminate pitch change with power change. On a typical airplane with the engine mounted in the nose, downthrust is typically used to counteract pitch-up with throttle-up. A wing mounted to the fuselage with a high angle of incidence in relation to the aircraft's centerline and stabilizer incidence can cause a major pitch-up as the aircraft increases speed. Adding downthrust counteracts that tendency, and vice-versa.

Typically, older models, originally designed for much lower power than we now use, and possibly designed for rudder-only, had a setup that made the model tend to climb. You adjusted the rate of climb by adjusting the wing/elevator incidence angles along with the thrust line. You moderated altitude gain by turning. Elevator control, if you had it, was modest, and allowed barely enough control to round out a descent to landing.

Downthrust doesn't pull the airplane down and "increase" the angle of the relative wind to the wing. Downthrust causes a nose-down pitching force that counteracts the pitch-up tendency of the wing/stab incidence relationship. If adding upthrust moderated a model's behavior with power change, then other issues were involved.