Try taking a clean white tissue and wiping your car windshield after a rainy day. The dirt left behind is evident. Leaving the dirt to accumulate, it will slowly but surely combine with moisture or other oily substances from the surroundings to form a denser layer of dirt. Regular cleanings will prevent the buildup of dirt. Sap and bird droppings are very destructive. Left to bake in the sun, permanent etching or deep scratching will cause critical damage.