This Front undercar Lower Control Arm Brace from Whiteline is designed specifically for the Subaru WRX and STi. It is designed to reduce front cross member flex during corner loading reducing alignment angle changes.
The Impreza did not come with a front subframe. While this does reduce weight it also allow the lower suspension mounts to flex in relation to each other. This flex or deflection means that all the points where the suspension arms bolt up are moving about under strenuous load. This movement is a bad thing as it detracts from the handling of your car.