The idea is that, while visibility in the rain through the front and rear screens is taken care of by the wipers, until now there's been nothing like that for the other windows and the exterior rear-view mirrors. As we all know, they can easily be obscured by rain and spray, especially on motorways, seriously restricting the driver's all-round view, particularly to the rear quarters.
Volvo's WRG side windows - in laminated glass - don't let the water smear across them, but are treated so that it forms "tiny pearls of moisture" which are blown off as the car gathers speed. In the case of the door mirrors, the effect is the same but the method is different, because the water is spread evenly across the glass surface, with no ripples. Volvo says that in really bad weather heated door mirrors will speed up the process.
The side window coating ought to last at least five years, and can be easily replaced, while the mirror treatment is reckoned to last the life of the car.
The WRG windows and mirrors are available on the latest S60, V70, S80, XC70 and XC90.