Q: What do rear traction arms and trailing arms actually do?

They locate the rear wheel relative to the chassis and control fore-aft movement every time you accelerate, brake, or turn. On a Model 3 or Model Y the traction and trailing arms are the structural backbone of the rear suspension. When the factory rubber bushings in those links flex, you feel it as vague turn-in and a rear end that settles instead of planting. Replacing them with rigid billet arms sharpens that response.