A short explanation of why you need an aftermarket sway bar is that it helps improve your vehicle’s handing and gives you, the driver, more control of the car. For example, if you're taking a right hand turn at high speed, the car begins to lean to the left and put more load on the left side tires, whereas the tires on the right hand side of the car start to lose contact with the road surface. Because of this, they aren't able to work effectively and efficiently in getting the car to rotate through the turn.