only for when you start out,limits torque to the rear wheels and starts you off in second gear,to get you moving on ice or snow.Thus it does not say R for rain.If you are loosing traction and hydroplaning on rain slick roads and the light is lighting up,you are exceeding the limits of traction of the tires and overdriving the road conditions.Slow down!!!!! W mode will do nothing for you Except for a few brief seconds move you at a crawl away from the stop light.
ask her how happy she is with her bmw in about a year ,when it has spent a few months at the dealer for various repairs.................