What benefit would off-camera flashes have for light painting for automotive (cars and bikes) photography?
Off-camera flash in relation to Light Painting would be very useful, especially if the flash power output can be adjusted. In fact, I occasionally use off-camera flash for that very purpose. Most flashes would typically be more powerful than an average torch/flashlight lamp, if that's what you need, and one with adjustable settings probably even more useful.
Off-camera flash is also useful for car and bike photography, because it gives greater flexibility than flash built-into camera, in terms of lighting angles and exposure options. It will enable you to have the flash positioned at particular angles to the car/bike, thus achieveing a more 3-dimensional effect, to highlight the car/bike shapes and contours, which may be important. And also for exposure control, off-camera flash will allow you to achieve more exposure options by positioning the flash closer to (or further from) the car/bike, which will enhance your exposure options.
You will probably need to experiment with the flash/camera exposure settings and flash lighting angles to get just the effect you're seeking.