Whether it's for performance or economy depends on how it's tuned and how the engine is sized relative to the equivalent pure gas car. The attractive things ab

Whether it's for performance or economy depends on how it's tuned and how the engine is sized relative to the equivalent pure gas car. The attractive things about these mild hybrid systems is that they're cheap, they don't weigh much (so you're not hurting efficiency by hauling around a bunch of batteries and electric motors), and they give you a lot of what a full hybrid (non-plug in) offers. If you're not plugging in, you don't need a big battery, because the hybrid only needs to store the energy from braking, and then deliver it when the car speeds up again. Even though the motor is only 21hp, it provides a lot of low-end torque, which lets you have good acceleration from a stop. This is something modern efficient gas engines are generally bad at, and the immediate acceleration in a lot of modern cars is even worse because of aggressive upshifting to save fuel. The electric motor papers over this deficiency, so you can be more aggressive about using a smaller engine with less low-end torque. The electric motor is also much more powerful than a normal starter motor, so it allows the car to more aggressively shut off the engine at stop lights and even when coasting, and turn it back on very quickly.