Hydrophobic coatings, including silicone-based coatings, lower the surface energy and smooth out the glass surface. These coatings repel water by reducing its polarity and in turn it's surface energy. Also, the coatings fill in and reduce microscopic inconsistencies in the glass. These factors force the water to bead and roll off your coated windshield. Products such as Rain-X Original Glass Water Repellent improve visibility by helping keep the windshield clearer during rain.