Frost or ice on your windscreen can affect your ability to properly see your surroundings. Removing ice or frost from glass will increase not only your own safety, but also that of others. It can also be illegal to drive without proper visibility.
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.