Mainly downthrust is to compensate for the fact that most of us are too lazy to use the elevator trim control as a primary control much like they do on full sized stuff. Add enough downthrust and the thrust vector makes up for the extra speed and the model only wants to climb a bit at full throttle and go into a powered glide at low throttle rather than try to loop or to dive like it would if the downthrust was not there and you didn't constantly play with the elevator trim.

Right thrust is just a way to make up for the torque reaction.

If you don't quite get the angles right don't sweat it. But I would not suggest taking it all out or you'll find that you NEED to learn to use the rudder and elevator trims as a basic and constantly used flight control.

Full sized stuff sets the throttle and basically trims the plane and flies for hours at a time in one direction and one trim. However our models are all over the sky and the throttle is on and off like a light switch at a rock concert. So we try to compensate by using thrust angles.