When glass has a temperature that is lower then, or equal to 0 degrees Celsius, any water coming in contact with it, will freeze and turn into ice. This is often seen in cold climates or seasons and might require the ice to be removed.
A rain repellent is a special product that is designed to be applied on glass. The product leaves a very hydrophobic layer on top of the glass. Water won’t be able to form any wetting, and the round beads are more easily blown off the glass.