With even the smallest of cracks in your windshield water can get though. Things like snow, ice, washer fluid, or even washing your car can cause a crack to spread.
Having a crack in your windshield can weaken the structural integrity of your windshield if water gets between the layers. Your windshield is made of two layer of glass and with a layer of protective vinyl in the middle. When cracked, this will leave space for moisture to get in-between the layers and cause more damage. Moisture can easily turn a simple windshield repair into a windshield replacement if you leave the damage long enough.