Power Parts are needed to power most of the means of travel. You need them to power wheels, balloons, and many other objects. For the ones that don't necessarilly 'power' the parts, such as the jets, they are there to provide speed. The jets are in truth the only power parts that can make your vehicle actually keep in the air (when the vehicle is pointed up). When you are using a large jet, then it is a very wise idea to have a Replenisher.