Increasing the front swaybar size on your Impreza helps to reduce the amount of suspension compression or "roll" the car see in corners. By reducing the roll, you'll be keeping your suspension in a better camber curve which in turn results in better front traction. This helps reduce some of the understeer these cars are known to exhibit in stock form. The adjustment allows you to dial in the bar to suit your driving style, or other modifications you may have made