Electric engines are not as efficient at 1 rpm as at 10000.
My Chevrolet Volt has 2 electrical engines. 1 150hp main engine and a smaller +/- 70hp (can't remember) second engine. When the battery is empty the 3rd engine (combustion) uses the 2nd engine as a power generator, charging the battery while you keep driving on the first main engine.
When the battery is full only the first engine is used, except when you go faster than +/- 100km/h. The main engine looses efficiency at higher rpm so they throttle the main engine back and use the second engine as well to provide trust. They say this gives a 15% efficiency boost.