This can be a tactically useful addition to your car and is not considered a weapon for the purposes of Races. It takes space like a weapon though.
The Rocket Booster is a rocket engine strapped onto your vehicle. When activated, it produces tremendous thrust in the direction opposite to the side of the vehicle it's mounted on. In practice, there's really no reason to mount it anywhere other than the rear. Unlike the Nitrous Booster, it will not dirty your engine. Activating a Rocket Booster while another vehicle is right up against your rear bumper can cause some minor damage to the trailing vehicle.
Using the Rocket Booster while turning can easily send you out of control. The overall effectiveness depends on the weight of the vehicle.