Right thrust is just as simple as down thrust. "Simple" you say???? None of this is really simple, is it

It's used for other things, but lots of times it's to cure the tendency of a model to turn left when it's pitched up. Where do we see that most often? and with really bad effect? On takeoffs.

There's this thing called p-effect. It happens with props, thus the "p" in the word p-effect. When a prop that's buzzing along level is pitched, it's no longer heading into the air perpendicular to the flow of that air. And one side of it sees an increased angle of attack, while the other side sees a decreased angle. (That'd be the right side and left side if you pitched the nose up on takeoff.) And the side seeing the increased angle of attack would pull better than the other side. And the prop would try to turn the airplane while you were trying to get it to pitch.

You get yaw when you're wanting pitch.

Since we most often fly right side up, and we most often see bad things from p-effect when we're doing things that pitch the nose up, we've most often built in some right thrust to cure that problem.

Best solution overall? Nope, but it's what has evolved.