It helps sell the car nothing much else, 30 years ago a car with rear window wipers was unheard of, so what if the back window gets wet, now like wow we need a rear window wiper (no you really don't). It sells like a 5 speaker sound system in your car, 2 speakers worked quite fine , or tinted windows, why tinted for on a cloudy or rainy day you can hardly see thru the tinting, great addition. Its all about adding features you dont really need just to sell a car to you, read your car brochure and see all the added features like, sunglass holders, umbrella slot ( yes most cars do have a place for a floding umbrella, just you never really noticed it, you know funny little holes all over the place, they are the added features.
Some cars do not have wiper blades on the windows because their unique designs. Some other cars have the wipers because they have tinted windows and if water clogs on them, the driver may be unable to have a clear vision on the sides.