They usually replace all the rubber contact points with metal ones to hold the engine in place better, and once you're doing that you might as well throw in a fun looking strut bar too. It makes the car a little more responsive and provides a lot more driver feedback from the engine. On the other hand that feedback means the car shakes like crazy on idle, so red light's can be a little painful, in an auto it's even recommended to shift to neutral to reduce the shaking.